Define GC_VSNPRINTF internal macro in misc.c (code refactoring)

* misc.c (GC_VSNPRINTF): New macro (defined to vsnprintf by default
and used by GC_PRINTF_FILLBUF).
Fix GC_INLINE definition to comply with ISO C90 standard (GCC)

* include/private/gc_priv.h (GC_INLINE): Define as "static __inline"
for GCC if "strict ISO/ANSI C90" mode is enforced.