PushPack struct os dependant

#ifdef __GNUC__
/*code for GNU C compiler */
	#define PRE_PACKED_ENUM_DEF
	#define POST_PACKED_ENUM_DEF __attribute__((packed));
#elif _MSC_VER
/*code specific to MSVC compiler*/
	#define PRE_PACKED_ENUM_DEF __pragma(pack(push, 1))
	#define POST_PACKED_ENUM_DEF __pragma(pack(pop));
#elif __MINGW32__
/*code specific to mingw compilers*/
	#define PRE_PACKED_ENUM_DEF
	#define POST_PACKED_ENUM_DEF __attribute__((packed));
#else
	#error "compiler not supported : see OsDef.hpp to fix"
#endif