Pass -isysroot to compiler / linker when doing a macOS build

Previously the compiler would use system headers located at
/System/Library/Frameworks, which could result in compilation failures
due to the headers not always being up-to-date in regards to the
latest installed macOS SDK headers that come with Xcode.

Fix the issue by passing the SDK path via the -isysroot option to the
compiler and linker invocations.

If no custom SDK path is given, the build system queries the SDK path
via xcrun --show-sdk-path, which returns something similar to

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/
 /Developer/SDKs/MacOSX.sdk/

Querying via xcrun is now also done for iphone (and simulator)
platforms as well.

Here is an example of a compilation failure message due to outdated
headers:

platform/osx/os_osx.mm:1421:41: error: use of undeclared identifier 'NSAppKitVersionNumber10_12'; did you mean 'NSAppKitVersionNumber'?
                                if (floor(NSAppKitVersionNumber) >= NSAppKitVersionNumber10_12) {
                                                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~
                                                                    NSAppKitVersionNumber
/System/Library/Frameworks/AppKit.framework/Headers/NSApplication.h:26:28: note: 'NSAppKitVersionNumber' declared here

2 files changed, 11 insertions, 4 deletions

diff --git a/platform/iphone/detect.py b/platform/iphone/detect.py
index b13a1e9643..8da7117077 100644
--- a/platform/iphone/detect.py
+++ b/platform/iphone/detect.py
@@ -1,7 +1,7 @@
 import os
 import string
 import sys
-
+from methods import detect_darwin_sdk_path
 
 def is_active():
     return True
@@ -22,9 +22,8 @@ def can_build():
 def get_opts():
     from SCons.Variables import BoolVariable
     return [
-        ('IPHONEPLATFORM', 'Name of the iPhone platform', 'iPhoneOS'),
         ('IPHONEPATH', 'Path to iPhone toolchain', '/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain'),
-        ('IPHONESDK', 'Path to the iPhone SDK', '/Applications/Xcode.app/Contents/Developer/Platforms/${IPHONEPLATFORM}.platform/Developer/SDKs/${IPHONEPLATFORM}.sdk/'),
+        ('IPHONESDK', 'Path to the iPhone SDK', ''),
         BoolVariable('game_center', 'Support for game center', True),
         BoolVariable('store_kit', 'Support for in-app store', True),
         BoolVariable('icloud', 'Support for iCloud', True),
@@ -103,13 +102,15 @@ def configure(env):
     ## Compile flags
 
     if (env["arch"] == "x86" or env["arch"] == "x86_64"):
-        env['IPHONEPLATFORM'] = 'iPhoneSimulator'
+        detect_darwin_sdk_path('iphonesimulator', env)
         env['ENV']['MACOSX_DEPLOYMENT_TARGET'] = '10.9'
         arch_flag = "i386" if env["arch"] == "x86" else env["arch"]
         env.Append(CCFLAGS=('-arch ' + arch_flag + ' -fobjc-abi-version=2 -fobjc-legacy-dispatch -fmessage-length=0 -fpascal-strings -fblocks -fasm-blocks -isysroot $IPHONESDK -mios-simulator-version-min=9.0 -DCUSTOM_MATRIX_TRANSFORM_H=\\\"build/iphone/matrix4_iphone.h\\\" -DCUSTOM_VECTOR3_TRANSFORM_H=\\\"build/iphone/vector3_iphone.h\\\"').split())
     elif (env["arch"] == "arm"):
+        detect_darwin_sdk_path('iphone', env)
         env.Append(CCFLAGS='-fno-objc-arc -arch armv7 -fmessage-length=0 -fno-strict-aliasing -fdiagnostics-print-source-range-info -fdiagnostics-show-category=id -fdiagnostics-parseable-fixits -fpascal-strings -fblocks -isysroot $IPHONESDK -fvisibility=hidden -mthumb "-DIBOutlet=__attribute__((iboutlet))" "-DIBOutletCollection(ClassName)=__attribute__((iboutletcollection(ClassName)))" "-DIBAction=void)__attribute__((ibaction)" -miphoneos-version-min=9.0 -MMD -MT dependencies'.split())
     elif (env["arch"] == "arm64"):
+        detect_darwin_sdk_path('iphone', env)
         env.Append(CCFLAGS='-fno-objc-arc -arch arm64 -fmessage-length=0 -fno-strict-aliasing -fdiagnostics-print-source-range-info -fdiagnostics-show-category=id -fdiagnostics-parseable-fixits -fpascal-strings -fblocks -fvisibility=hidden -MMD -MT dependencies -miphoneos-version-min=9.0 -isysroot $IPHONESDK'.split())
         env.Append(CPPFLAGS=['-DNEED_LONG_INT'])
         env.Append(CPPFLAGS=['-DLIBYUV_DISABLE_NEON'])
diff --git a/platform/osx/detect.py b/platform/osx/detect.py
index 8a0883eca3..72a57b4e87 100644
--- a/platform/osx/detect.py
+++ b/platform/osx/detect.py
@@ -1,5 +1,6 @@
 import os
 import sys
+from methods import detect_darwin_sdk_path
 
 
 def is_active():
@@ -23,6 +24,7 @@ def get_opts():
 
     return [
         ('osxcross_sdk', 'OSXCross SDK version', 'darwin14'),
+        ('MACOS_SDK_PATH', 'Path to the macOS SDK', ''),
         EnumVariable('debug_symbols', 'Add debugging symbols to release builds', 'yes', ('yes', 'no', 'full')),
         BoolVariable('separate_debug_symbols', 'Create a separate file containing debugging symbols', False),
     ]
@@ -84,6 +86,10 @@ def configure(env):
             env['AS'] = mpprefix + "/libexec/llvm-" + mpclangver + "/bin/llvm-as"
             env.Append(CCFLAGS=['-D__MACPORTS__']) #hack to fix libvpx MM256_BROADCASTSI128_SI256 define
 
+        detect_darwin_sdk_path('osx', env)
+        env.Append(CPPFLAGS=['-isysroot', '$MACOS_SDK_PATH'])
+        env.Append(LINKFLAGS=['-isysroot', '$MACOS_SDK_PATH'])
+
     else: # osxcross build
         root = os.environ.get("OSXCROSS_ROOT", 0)
         basecmd = root + "/target/bin/x86_64-apple-" + env["osxcross_sdk"] + "-"