Add -fpermissive to gcc cflags

We again have code that does not compile with gcc:

../../../src/CppParser/AST.h:770:19: error: declaration of
‘CppSharp::CppParser::AST::MacroLocation
CppSharp::CppParser::AST::PreprocessedEntity::MacroLocation’
[-fpermissive]
     MacroLocation MacroLocation;
                   ^
../../../src/CppParser/AST.h:756:12: error: changes meaning of
‘MacroLocation’ from ‘enum class
CppSharp::CppParser::AST::MacroLocation’ [-fpermissive]
 enum class MacroLocation

Instead of trying to figure out a good name for the property, and to
avoid the same issues in the future, let's just add the -fpermissive
flag back.

Maybe it can be removed in the future if and when Linux support works
well and is supported by the build test bot.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@iki.fi>

build/Helpers.lua

@@ -19,7 +19,7 @@ gendir = path.join(builddir, "gen");
 
 common_flags = { "Unicode", "Symbols" }
 msvc_buildflags = { "/wd4267" }
-gcc_buildflags = { "-std=c++11" }
+gcc_buildflags = { "-std=c++11 -fpermissive" }
 
 msvc_cpp_defines = { }