[WIP] Use cmake modules for PhysFS

[frabert]

This PR should solve issue REGoth-project/REGoth#369 by removing the need to keep our git clone of the PhysFS mercurial repo.

Instead, it searches for a preinstalled copy of PhysFS in the system.
Issues with this method:

• Windows doesn't have a central source for packages, PhysFS will need to be compiled and installed manually
• Ubuntu before version 18.04 ships an obsolete version of PhysFS, so it will need to be manually compiled and installed, too

Both of the issues can be solved by distributing a script with REGoth which automatically downloads and installs the dependencies in a subdirectory, as discussed previously with @markusobi

[ataulien]

Done in #33.