Require GMP and take GAP's GMP_PREFIX into account

[fingolfin]

There is no good reason to allow building anupq without GMP
these days. On the downside, if one fails to do that (which may
happen without the user even knowing what happened), one may get
unexpected errors or perhaps even wrong results. So better to
just require this.

This also simplifies the logic in configure.ac considerably,
and allows us to just use the same as in anunq.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 17.17%. Comparing base (368ec0b) to head (fc2d923).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   16.71%   17.17%   +0.46%     
==========================================
  Files         146      145       -1     
  Lines       12840    12737     -103     
  Branches     1936     1929       -7     
==========================================
+ Hits         2146     2188      +42     
+ Misses      10686    10539     -147     
- Partials        8       10       +2     

see 15 files with indirect coverage changes