Allow mupdf (and other dependencies) to be linked instead of building from source

[dit7ya]

Is your feature request related to a problem? Please describe.
I am trying to package this for Nixpkgs and as nixpkgs already has a fairly complicated build procedure for the mupdf dependency, it is quite a lot of work to get this incorporated inside the librum package.

Describe the solution you'd like
Allow some cmake flags for all the mupdf libs required. (NOTE: for it to work under NixOS, it cannot assume FHS as NixOS doesn't adhere to that)

Describe alternatives you've considered
None

Additional context
This is how complex the mupdf derivation for Nixpkgs is - https://github.com/NixOS/nixpkgs/tree/420b97bf42e851f688dbc343c500fbed4dd2e26b/pkgs/applications/misc/mupdf

[DavidLazarescu]

We have a branch for enabling this already. We are working on it, and it will (probably) be in the next version of Librum.