SCons: Review uses of CCFLAGS, CXXFLAGS and CPPFLAGS

Many contributors (me included) did not fully understand what CCFLAGS,
CXXFLAGS and CPPFLAGS refer to exactly, and were thus not using them
in the way they are intended to be.

As per the SCons manual: https://www.scons.org/doc/HTML/scons-user/apa.html

- CCFLAGS: General options that are passed to the C and C++ compilers.
- CFLAGS: General options that are passed to the C compiler (C only;
  not C++).
- CXXFLAGS: General options that are passed to the C++ compiler. By
  default, this includes the value of $CCFLAGS, so that setting
  $CCFLAGS affects both C and C++ compilation.
- CPPFLAGS: User-specified C preprocessor options. These will be
  included in any command that uses the C preprocessor, including not
  just compilation of C and C++ source files [...], but also [...]
  Fortran [...] and [...] assembly language source file[s].

TL;DR: Compiler options go to CCFLAGS, unless they must be restricted
to either C (CFLAGS) or C++ (CXXFLAGS). Preprocessor defines go to
CPPFLAGS.

1 files changed, 11 insertions, 9 deletions

diff --git a/SConstruct b/SConstruct
index 6bb129a174..e943f8dc04 100644
--- a/SConstruct
+++ b/SConstruct
@@ -161,8 +161,8 @@ opts.Add("CXX", "C++ compiler")
 opts.Add("CC", "C compiler")
 opts.Add("LINK", "Linker")
 opts.Add("CCFLAGS", "Custom flags for both the C and C++ compilers")
-opts.Add("CXXFLAGS", "Custom flags for the C++ compiler")
 opts.Add("CFLAGS", "Custom flags for the C compiler")
+opts.Add("CXXFLAGS", "Custom flags for the C++ compiler")
 opts.Add("LINKFLAGS", "Custom flags for the linker")
 
 # add platform specific options
@@ -271,17 +271,18 @@ if selected_platform in platform_list:
 
     CCFLAGS = env.get('CCFLAGS', '')
     env['CCFLAGS'] = ''
-
     env.Append(CCFLAGS=str(CCFLAGS).split())
 
     CFLAGS = env.get('CFLAGS', '')
     env['CFLAGS'] = ''
-
     env.Append(CFLAGS=str(CFLAGS).split())
 
+    CXXFLAGS = env.get('CXXFLAGS', '')
+    env['CXXFLAGS'] = ''
+    env.Append(CXXFLAGS=str(CXXFLAGS).split())
+
     LINKFLAGS = env.get('LINKFLAGS', '')
     env['LINKFLAGS'] = ''
-
     env.Append(LINKFLAGS=str(LINKFLAGS).split())
 
     flag_list = platform_flags[selected_platform]
@@ -322,15 +323,16 @@ if selected_platform in platform_list:
             # FIXME: enable -Wlogical-op and -Wduplicated-branches once #27594 is merged
             # Note: enable -Wimplicit-fallthrough for Clang (already part of -Wextra for GCC)
             # once we switch to C++11 or later (necessary for our FALLTHROUGH macro).
-            env.Append(CCFLAGS=['-Wall', '-Wextra', '-Wno-unused-parameter',
-                '-Wctor-dtor-privacy', '-Wnon-virtual-dtor']
+            env.Append(CCFLAGS=['-Wall', '-Wextra', '-Wno-unused-parameter']
                 + all_plus_warnings + shadow_local_warning)
+            env.Append(CXXFLAGS=['-Wctor-dtor-privacy', '-Wnon-virtual-dtor'])
             if methods.using_gcc(env):
-                env['CCFLAGS'] += ['-Wno-clobbered', '-Walloc-zero', '-Wnoexcept',
-                    '-Wduplicated-cond', '-Wplacement-new=1', '-Wstringop-overflow=4']
+                env.Append(CCFLAGS=['-Wno-clobbered', '-Walloc-zero',
+                    '-Wduplicated-cond', '-Wstringop-overflow=4'])
+                env.Append(CXXFLAGS=['-Wnoexcept', '-Wplacement-new=1'])
                 version = methods.get_compiler_version(env)
                 if version != None and version[0] >= '9':
-                    env['CCFLAGS'] += ['-Wattribute-alias=2']
+                    env.Append(CCFLAGS=['-Wattribute-alias=2'])
         elif (env["warnings"] == 'all'):
             env.Append(CCFLAGS=['-Wall'] + shadow_local_warning)
         elif (env["warnings"] == 'moderate'):