Protect C++0x definitions inside __GXX_EXPERIMENTAL_CXX0X__

When compiling in C++0x/11 mode, GCC defines the preprocessor macro

__GXX_EXPERIMENTAL_CXX0X__

FNALCore's macro definitions of support for C++0x/11 features need
to be activated only when this is defined, otherwise upstream
errors may occur. This was discovered when trying to build Reflex
dictionaries using GCCXML.

Wrap C++11 support macros inside a #if defined block on the
definition of __GXX_EXPERIMENTAL_CXX0X__

Update config header to use main cpp0x/compiler_macros.h header
to simplify and unify interfaces here.