Switch to GAP 4.13.0

[fingolfin]

No description provided.

[codecov]

Codecov Report

Merging #975 (34056e5) into master (32aaa36) will decrease coverage by 0.93%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #975      +/-   ##
==========================================
- Coverage   76.73%   75.80%   -0.93%     
==========================================
  Files          51       51              
  Lines        4195     4187       -8     
==========================================
- Hits         3219     3174      -45     
- Misses        976     1013      +37     

Files	Coverage Δ
gap/systemfile.g	94.44% <100.00%> (ø)
pkg/JuliaExperimental/PackageInfo.g	100.00% <100.00%> (ø)
pkg/JuliaExperimental/gap/numfield.g	95.24% <100.00%> (ø)
pkg/JuliaInterface/PackageInfo.g	90.17% <100.00%> (ø)
src/setup.jl	79.82% <ø> (-7.89%)	⬇️

... and 1 file with indirect coverage changes

[ThomasBreuer]

Looks good, as far as I can judge.
Just one possible simplification (see remark).
(And I assume that adjusting sysinfo["GAP_CFLAGS"] and sysinfo["GAP_CXXFLAGS"] had first been changed but now this is not needed anymore.)