Modernize ac_find_gap.m4

[fingolfin]

• drop CONFIGNAME support
• make it possible to compile against future GAP releases with compatibility mode turned off

[codecov[bot]]

Codecov Report

Merging #15 into master will increase coverage by 16.91%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #15       +/-   ##
===========================================
+ Coverage   63.06%   79.97%   +16.91%     
===========================================
  Files          14       24       +10     
  Lines        1397     2497     +1100     
===========================================
+ Hits          881     1997     +1116     
+ Misses        516      500       -16

Impacted Files	Coverage Δ
gap/anadata.gd	100% <0%> (ø)
gap/anatph.gd	100% <0%> (ø)
gap/location.gd	100% <0%> (ø)
gap/relator.gd	100% <0%> (ø)
gap/place.gd	100% <0%> (ø)
gap/examples.gd	100% <0%> (ø)
gap/util.gd	100% <0%> (ø)
gap/small_pregroups.gd	100% <0%> (ø)
gap/pregroup.gd	100% <0%> (ø)
gap/presentation.gd	100% <0%> (ø)
... and 13 more

[fingolfin]

@markuspf any objections to just merging this?