Use __gnu_inline__ for __extern_always_inline in g++-4.2

Use the __gnu_inline__ attribute in _FORTIFY_SOURCE's __extern_always_inline
macro whenever the compiler supports it. Previously this macro only included
the __gnu_inline__ attribute in C++ mode for gcc >= 4.3. However,
__gnu_inline__ semantics are always desired for the __extern_always_inline
functions, and are available in g++ 4.2 (and some releases of g++ 4.1, and
also in Clang, which claims to be g++ 4.2).

This change stops g++-4.2 from emitting weak definitions for the fortify
wrapper functions if they can't be inlined, and also improves Clang
compatibility.