More precise regexp to extract version line from 'gcc -v'.

Reported by Robert Buchholz <rbu@gentoo.org>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

xen/Makefile

@@ -87,7 +87,7 @@ include/xen/compile.h: include/xen/compile.h.in .banner
 	    -e 's/@@whoami@@/$(XEN_WHOAMI)/g' \
 	    -e 's/@@domain@@/$(XEN_DOMAIN)/g' \
 	    -e 's/@@hostname@@/$(shell hostname)/g' \
-	    -e 's!@@compiler@@!$(shell $(CC) $(CFLAGS) -v 2>&1 | grep -i "gcc.*version")!g' \
+	    -e 's!@@compiler@@!$(shell $(CC) $(CFLAGS) -v 2>&1 | grep -i "gcc .*version ")!g' \
 	    -e 's/@@version@@/$(XEN_VERSION)/g' \
 	    -e 's/@@subversion@@/$(XEN_SUBVERSION)/g' \
 	    -e 's/@@extraversion@@/$(XEN_EXTRAVERSION)/g' \