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
for `mod_php5` we use `apache.mod_php5.install_method`, can we standardize to that here as well?