configure: Keep -Werror enabled for Release Candidates

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>