Rewrite FindGMP to provide imported targets

Simon Praetoriusrequested to merge
feature/find_gmp into master

Summary

This MR adds imported-targets to the FindGMP.cmake module, following modern cmake guidelines.

Details

Instead of exporting include directories and libraries as variables, the find modules exports an imported target that contains all the necessary information to include and link GMP. Two such imported targets are provided: GMP::gmp and GMP::gmpxx for the C and C++ library, respectively.

Acknowledgements

This find-module is developed together with @lukas.riedel

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