skalibs
Software
www.skarnet.org
gccattributes.h is a set of wrappers around gcc attributes (duh). It defines macros that are always valid, and that have no effect if the compiler is not gcc or the used version of gcc does not support the wanted attribute.
For instance:
extern unsigned int str_len (char const *) gccattr_pure ;
defines the str_len function as pure if it is supported.
The src/headers/gccattributes.h is the only reliable reference.