[physfs] Option to disable CMake docs

[Mattparks]

I have a CMake project with PhysFS added as a sub-directory along with
other libraries, I am having a name conflict  with the docs custom target.
A great way to fix this would be to add a PHYSFS_BUILD_DOCS option to the
PhysFS CMakeLists file, and only find Doxygen if the option is enabled.
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