<regex.h>: reserve a regcomp field for REG_POSIX

For libc regcomp, this will be a nop. libregex will take this to mean that
it needs to turn off GNU extensions, effectively switching it back to the
POSIX-compliant libc implementation at runtime.

include/regex.h

@@ -71,6 +71,7 @@ typedef struct {
 #define	REG_NOSPEC	0020
 #define	REG_PEND	0040
 #define	REG_DUMP	0200
+#define	REG_POSIX	0400	/* only POSIX-compliant regex (libregex) */
 
 /* regerror() flags */
 #define	REG_ENOSYS	(-1)