There are two copies of `StrLen` in the RapidJSON code base
* strfunc.h: rapidjson::internal::StrLen<Ch>
* unittest.h: Strlen<Ch>
To hide a warning on MSVC, align both implementations to use
'unsigned/SizeType' as return type and add an explicit cast.
Warning push/pop support has been added to GCC in version 4.6.0,
and pragmas to ignore certain warnings are present since 4.2.0.
This patch hides the compiler-specific warning push/pop/disable
pragmas behind a macro-based implementation (currently for MSVC and
clang /GCC.
This avoids warnings, as seen e.g. on GCC 4.4:
../../include/rapidjson/document.h:14: error: expected [error|warning|ignored] after ‘#pragma GCC diagnostic’
and earlier versions complaining about unknown pragmas being ignored.
Note: unittest.h and perftest.h need to check for compilers
explicitly, as rapidjson.h is not included there.
