Remove werror for maintainer-mode

* configure.ac (CFLAGS, CXXFLAGS): Remove -Werror

Since it is better to show the error instead of
hiding it, as to not cause a compilation failue due
to -Werror, I have removed Werror and readded warnings
for: "deprecated-copy" "address" and "stringop-overflow".

The cause for this was that GPGME recently deprecated
Error::asString and depending on an unreleased
GPGME would be too early to fix the deprecation
warnings without IFDEFs