build: Use more compiler warnings
build: Use more compiler warnings

* configure.ac: Add useful compiler warnings.
--

It is strange that this seems to be the only GnuPG package which does
not use modern warning.

Signed-off-by: Werner Koch <13bd1fc5137f503b0a8355ad400e7a87ebb45b25@gnupg.org>

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme