configure: Keep -Werror enabled for Release Candidates

The automatic drop of -Werror during the RC phases has in the past led
to warnings creeping into submaintainer trees.

Last QEMU Summit it was concluded that -Werror should stay on and
enabled only as part of the release process.

To relieve our release manager, instead of always enabling -Werror or
doing some number magic, let's enable it depending on whether a .git/
directory exists in the source tree.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

configure

@@ -1180,7 +1180,7 @@ fi
 z_version=`cut -f3 -d. $source_path/VERSION`
 
 if test -z "$werror" ; then
-    if test "$z_version" = "50" -a \
+    if test -d "$source_path/.git" -a \
         "$linux" = "yes" ; then
         werror="yes"
     else