SCons: Use CPPDEFINES instead of CPPFLAGS for pre-processor defines

It's the recommended way to set those, and is more portable
(automatically prepends -D for GCC/Clang and /D for MSVC).

We still use CPPFLAGS for some pre-processor flags which are not
defines.

modules/webm/libvpx/SCsub

@@ -323,7 +323,7 @@ if webm_cpu_x86:
     elif cpu_bits == '64':
         env_libvpx["ASCPU"] = 'X86_64'
 
-    env_libvpx.Append(CPPFLAGS=['-DWEBM_X86ASM'])
+    env_libvpx.Append(CPPDEFINES=['WEBM_X86ASM'])
 
     webm_simd_optimizations = True
 
@@ -337,7 +337,7 @@ if webm_cpu_arm:
         env_libvpx["ASFLAGS"] = ''
         env_libvpx["ASCOM"] = '$AS $ASFLAGS -o $TARGET $SOURCES'
 
-    env_libvpx.Append(CPPFLAGS=['-DWEBM_ARMASM'])
+    env_libvpx.Append(CPPDEFINES=['WEBM_ARMASM'])
 
     webm_simd_optimizations = True