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
Required because the dependency from the `Makefile`(like `build/special.mk`) are not in the `lib_dir` (like it is on Rhel)