AC_GNU_SOURCE is broken, define _GNU_SOURCE unconditionally
AC_GNU_SOURCE is broken, define _GNU_SOURCE unconditionally

mgjv/libusual,markokr/libusual,rabbitt/libusual,ifduyue/libusual,markokr/libusual,markokr/libusual,mgjv/libusual,rabbitt/libusual,markokr/libusual,mgjv/libusual,ifduyue/libusual,rabbitt/libusual,mgjv/libusual,ifduyue/libusual,rabbitt/libusual,ifduyue/libusual