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
`build/special.mk` is not on Debian by default, we need to install this package to get it (it will then be placed in /usr/share/apache2)