fixing FindGMP.cmake on Debian/Buster

Alexander Voigtrequested to merge
Expander/firefly:fix-findgmp into master

On my machine (Debian/Buster) the FindGMP.cmake script could not find the GMP installation that I've installed via the distribution.

Here I'm proposing a rewrite of FindGMP.cmake where this issue is fixed. I've tested that it can also find locally build GMP installations.

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