Fix warning about functions defined but not used [-Wunused-function]

Fixes the following GCC 5 warnings:
```
core/io/zip_io.h:128:26: warning: 'zlib_filefunc_def zipio_create_io_from_file(FileAccess**)' defined but not used [-Wunused-function]
core/script_debugger_remote.cpp:110:17: warning: 'ObjectID safe_get_instance_id(const Variant&)' defined but not used [-Wunused-function]
drivers/unix/socket_helpers.h:103:12: warning: 'int _socket_create(IP::Type&, int, int)' defined but not used [-Wunused-function]
drivers/unix/socket_helpers.h:45:15: warning: 'size_t _set_sockaddr(sockaddr_storage*, const IP_Address&, int, IP::Type)' defined but not used [-Wunused-function]
drivers/unix/socket_helpers.h:76:15: warning: 'size_t _set_listen_sockaddr(sockaddr_storage*, int, IP::Type, IP_Address)' defined but not used [-Wunused-function]
editor/editor_fonts.cpp:40:24: warning: 'Ref<BitmapFont> make_font(int, int, int, int, const int*, const Ref<Texture>&)' defined but not used [-Wunused-function]
editor/editor_themes.cpp:85:26: warning: 'Ref<StyleBoxFlat> change_border_color(Ref<StyleBoxFlat>, Color)' defined but not used [-Wunused-function]
editor/import/editor_import_collada.cpp:493:13: warning: 'void _generate_normals(const PoolVector<int>&, const PoolVector<Vector3>&, PoolVector<Vector3>&)' defined but not used [-Wunused-function]
editor/import/editor_import_collada.cpp:524:13: warning: 'void _generate_tangents_and_binormals(const PoolVector<int>&, const PoolVector<Vector3>&, const PoolVector<Vector3>&, const PoolVector<Vector3>&, PoolVector<float>&)' defined but not used [-Wunused-function]
editor/pvrtc_compress.cpp:118:13: warning: 'void _compress_etc(Image*)' defined but not used [-Wunused-function]
modules/etc/image_etc.cpp:89:13: warning: 'void _decompress_etc1(Image*)' defined but not used [-Wunused-function]
modules/etc/image_etc.cpp:93:13: warning: 'void _decompress_etc2(Image*)' defined but not used [-Wunused-function]
modules/gdscript/editor/gdscript_highlighter.cpp:46:13: warning: 'bool _is_whitespace(CharType)' defined but not used [-Wunused-function]
scene/2d/cpu_particles_2d.cpp:510:14: warning: 'float rand_from_seed_m1_p1(uint32_t&)' defined but not used [-Wunused-function]
scene/3d/cpu_particles.cpp:474:14: warning: 'float rand_from_seed_m1_p1(uint32_t&)' defined but not used [-Wunused-function]
scene/resources/default_theme/default_theme.cpp:123:20: warning: 'Ref<Shader> make_shader(const char*, const char*, const char*)' defined but not used [-Wunused-function]
scene/resources/default_theme/default_theme.cpp:130:24: warning: 'Ref<BitmapFont> make_font(int, int, int, int, const int*, const Ref<Texture>&)' defined but not used [-Wunused-function]
```

Had to split `core/io/zip_io.h` into header and .cpp file without 'static' keyword.

Not fixed yet (static definition in header used in some files but not all):
```
modules/websocket/lws_helper.h:111:13: warning: 'void _lws_make_protocols(void*, int (*)(lws*, lws_callback_reasons, void*, void*, size_t), PoolVector<String>, _LWSRef**)' defined but not used [-Wunused-function]
```

Also fixed a couple other warnings missed in previous commits.

6 files changed, 8 insertions, 33 deletions

diff --git a/modules/etc/image_etc.cpp b/modules/etc/image_etc.cpp
index 57f5b68c61..fbbc765bf2 100644
--- a/modules/etc/image_etc.cpp
+++ b/modules/etc/image_etc.cpp
@@ -88,14 +88,6 @@ static Etc::Image::Format _image_format_to_etc2comp_format(Image::Format format)
 	}
 }
 
-static void _decompress_etc1(Image *p_img) {
-	// not implemented, to be removed
-}
-
-static void _decompress_etc2(Image *p_img) {
-	// not implemented, to be removed
-}
-
 static void _compress_etc(Image *p_img, float p_lossy_quality, bool force_etc1_format, Image::CompressSource p_source) {
 	Image::Format img_format = p_img->get_format();
 	Image::DetectChannels detected_channels = p_img->get_detected_channels();
@@ -245,8 +237,5 @@ static void _compress_etc2(Image *p_img, float p_lossy_quality, Image::CompressS
 void _register_etc_compress_func() {
 
 	Image::_image_compress_etc1_func = _compress_etc1;
-	//Image::_image_decompress_etc1 = _decompress_etc1;
-
 	Image::_image_compress_etc2_func = _compress_etc2;
-	//Image::_image_decompress_etc2 = _decompress_etc2;
 }
diff --git a/modules/gdscript/editor/gdscript_highlighter.cpp b/modules/gdscript/editor/gdscript_highlighter.cpp
index a1163b5d8d..c199667270 100644
--- a/modules/gdscript/editor/gdscript_highlighter.cpp
+++ b/modules/gdscript/editor/gdscript_highlighter.cpp
@@ -43,10 +43,6 @@ static bool _is_text_char(CharType c) {
 	return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') || c == '_';
 }
 
-static bool _is_whitespace(CharType c) {
-	return c == '\t' || c == ' ';
-}
-
 static bool _is_char(CharType c) {
 
 	return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_';
diff --git a/modules/gdscript/gdscript_editor.cpp b/modules/gdscript/gdscript_editor.cpp
index 55bc3d2359..ddd9e6b01c 100644
--- a/modules/gdscript/gdscript_editor.cpp
+++ b/modules/gdscript/gdscript_editor.cpp
@@ -51,12 +51,6 @@ void GDScriptLanguage::get_string_delimiters(List<String> *p_delimiters) const {
 	p_delimiters->push_back("\"\"\" \"\"\"");
 }
 Ref<Script> GDScriptLanguage::get_template(const String &p_class_name, const String &p_base_class_name) const {
-#ifdef TOOLS_ENABLED
-	bool th = EDITOR_DEF("text_editor/completion/add_type_hints", false);
-#else
-	bool th = false;
-#endif
-
 	String _template = "extends %BASE%\n"
 					   "\n"
 					   "# Declare member variables here. Examples:\n"
diff --git a/modules/stb_vorbis/SCsub b/modules/stb_vorbis/SCsub
index 897d05961c..d14939a3b1 100644
--- a/modules/stb_vorbis/SCsub
+++ b/modules/stb_vorbis/SCsub
@@ -3,8 +3,14 @@
 Import('env')
 Import('env_modules')
 
+env_stb_vorbis = env_modules.Clone()
+
 # Thirdparty source files
+thirdparty_sources = ["#thirdparty/misc/stb_vorbis.c"]
 
-env_stb_vorbis = env_modules.Clone()
+env_thirdparty = env_stb_vorbis.Clone()
+env_thirdparty.disable_warnings()
+env_thirdparty.add_source_files(env.modules_sources, thirdparty_sources)
 
+# Godot's own source files
 env_stb_vorbis.add_source_files(env.modules_sources, "*.cpp")
diff --git a/modules/stb_vorbis/audio_stream_ogg_vorbis.cpp b/modules/stb_vorbis/audio_stream_ogg_vorbis.cpp
index 57b6b5343e..5dbe0b4b00 100644
--- a/modules/stb_vorbis/audio_stream_ogg_vorbis.cpp
+++ b/modules/stb_vorbis/audio_stream_ogg_vorbis.cpp
@@ -32,11 +32,6 @@
 
 #include "core/os/file_access.h"
 
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
-#include "thirdparty/misc/stb_vorbis.c"
-#pragma GCC diagnostic pop
-
 void AudioStreamPlaybackOGGVorbis::_mix_internal(AudioFrame *p_buffer, int p_frames) {
 
 	ERR_FAIL_COND(!active);
diff --git a/modules/stb_vorbis/audio_stream_ogg_vorbis.h b/modules/stb_vorbis/audio_stream_ogg_vorbis.h
index 71a957a6af..8b42111847 100644
--- a/modules/stb_vorbis/audio_stream_ogg_vorbis.h
+++ b/modules/stb_vorbis/audio_stream_ogg_vorbis.h
@@ -34,12 +34,7 @@
 #include "core/io/resource_loader.h"
 #include "servers/audio/audio_stream.h"
 
-#define STB_VORBIS_HEADER_ONLY
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
-#include "thirdparty/misc/stb_vorbis.c"
-#pragma GCC diagnostic pop
-#undef STB_VORBIS_HEADER_ONLY
+#include "thirdparty/misc/stb_vorbis.h"
 
 class AudioStreamOGGVorbis;