Fix compile problem on OS X

SConstruct

@@ -66,7 +66,7 @@ elif target_platform == 'windows':
         env.Append(CCFLAGS = [ '-g', '-O3', '-std=c++14', '-Wwrite-strings' ])
         env.Append(LINKFLAGS = [ '--static', '-Wl,--no-undefined', '-static-libgcc', '-static-libstdc++' ])
 
-elif platform == 'osx':
+elif target_platform == 'osx':
     if ARGUMENTS.get('use_llvm', 'no') == 'yes':
         env['CXX'] = 'clang++'