Add a flag to allow to install mod_fastcgi from source (even on Debian familly)

You might need to install mod_fastcgi from source, even if you use Debian and have the mod_fastcgi package.
The PR is adding unit and integration tests for mod_fastcgi as well
We still need `apache.libexec_dir` for other things. It looks like debian refers to full build dir, so maybe we want this to be called `apache.build_dir` pointing towards `/usr/share/apache2/build`?