Update cmake find modules

Supersedes #310.
I was looking up if we should use `GMP_INCLUDE_DIRS` or `GMP_INCLUDE_DIRS`, and the convention seems to be to use `GMP_INCLUDE_DIRS`, so I would just stick to that: https://github.com/Kitware/CMake/blob/861b0df4d1b11e6fbdc26c07fe6101f02a38b3f6/Help/manual/cmake-developer.7.rst#standard-variable-names