Don't lower security standards with gcc 4.9

For some reason since e4d65c07cee90461caad8b24f069fc93a82c3877
we started compiling -Wno-format, possibly because of plugins
dependencies not compiling.
Fedora (and Debian to some degrees) are starting to default to
-Wformat -Werror=format-security which is something sensible for
uwsgi. So let the distro handle the situation when a very recent
gcc version is encountered since they probably know better.
Merge pull request #759 from xrmx/formatsec

Don't lower security standards with recent gcc