Stop caring about GCC versions between 2.5 and 2.7, they are

Stop caring about GCC versions between 2.5 and 2.7, they are
historic artefacts.
Remove a double define of __func__ from the !LINT case.
Define __func__ to NULL for GCC before 2.7 and non ISO C99 compilers.
Since all GCC versions above 2.95 support __restrict, include them
in the check too. Remove the weak defines for !__STDC__, we don't
support such a setting.
If _C99_SOURCE is set, don't include POSIX / BSD / XSI extensions.

Partly-submitted-by: Andreas Hauser <andy@splashground.de>