build: Use -Werror only for the check.

* configure.ac: Do not add -Werror to mycflags.

On Windows and possible also on other platforms we expect to a get a
few errors or warnins. Thus we can't use -Werror by default. This is
why we have a separate configure options --enable-werror ;-).

• Fixes-commit: 3ecd1a41be7c880976987d13e88342c98f37e064
• Signed-off-by: Werner Koch <wk@gnupg.org>