bsdgrep: switch to libregex for GNU_GREP_COMPAT

libregex is incomplete, but it's a bit less buggy than the in-base
libgnuregex and mostly OK.

While here, rename -DIWTH_GNU -> -DWITH_GNU_COMPAT; the option implies
that we're compatible with the GNU counterpart, not that we're including GNU
anything.

usr.bin/grep/Makefile

@@ -61,8 +61,8 @@ MLINKS+= grep.1 egrep.1 \
 .endif
 
 .if ${MK_GNU_GREP_COMPAT} != "no"
-CFLAGS+= -I${SYSROOT:U${DESTDIR}}/usr/include/gnu -DWITH_GNU
-LIBADD+=	gnuregex
+CFLAGS+=	-DWITH_GNU_COMPAT
+LIBADD+=	regex
 .endif
 
 HAS_TESTS=

usr.bin/grep/grep.c

@@ -555,7 +555,7 @@ main(int argc, char *argv[])
 			filebehave = FILE_MMAP;
 			break;
 		case 'V':
-#ifdef WITH_GNU
+#ifdef WITH_GNU_COMPAT
 			printf(errstr[9], getprogname(), VERSION);
 #else
 			printf(errstr[8], getprogname(), VERSION);