expose FNM_* extensions in fnmatch.h without _GNU_SOURCE

at least a couple of these are used on BSD too, and the FNM_*
namespace is reserved in fnmatch.h anyway.

include/fnmatch.h

@@ -10,12 +10,9 @@ extern "C" {
 #define	FNM_PATHNAME 0x1
 #define	FNM_NOESCAPE 0x2
 #define	FNM_PERIOD   0x4
-
-#ifdef _GNU_SOURCE
 #define	FNM_LEADING_DIR	0x8           
 #define	FNM_CASEFOLD	0x10
 #define	FNM_FILE_NAME	FNM_PATHNAME
-#endif
 
 #define	FNM_NOMATCH 1
 #define FNM_NOSYS   (-1)