declare basename in string.h when _GNU_SOURCE is defined
declare basename in string.h when _GNU_SOURCE is defined

note that it still will have the standards-conformant behavior, not
the GNU behavior. but at least this prevents broken code from ending
up with truncated pointers due to implicit declarations...

chardi/musl,davidlazar/musl,chardi/musl,davidlazar/musl,chardi/musl