Rewrite FindGMP to provide imported targets
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