buildsys: cleanup configure.ac

[fingolfin]

• disable unusued automake dist mode
• use GAP from sysinfo.gap instead of ${GAP_BIN_DIR}/bin/gap.sh to better support future versions of GAP that can be installed system wide using make install
• remove some redundant checks for system headers

[codecov[bot]]

Codecov Report

Merging #49 (1a7c157) into master (0913786) will decrease coverage by 13.57%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #49       +/-   ##
===========================================
- Coverage   49.25%   35.67%   -13.58%     
===========================================
  Files         147      147               
  Lines       12901    12901               
===========================================
- Hits         6354     4603     -1751     
- Misses       6547     8298     +1751     

Impacted Files	Coverage Δ
src/step_range.c	0.00% <0.00%> (-100.00%)	:arrow_down:
src/update_name.c	0.00% <0.00%> (-100.00%)	:arrow_down:
src/close_subgroup.c	0.00% <0.00%> (-100.00%)	:arrow_down:
src/assemble_matrix.c	0.00% <0.00%> (-100.00%)	:arrow_down:
src/find_permutation.c	0.00% <0.00%> (-100.00%)	:arrow_down:
src/strip_identities.c	0.00% <0.00%> (-100.00%)	:arrow_down:
src/subgroup_to_label.c	0.00% <0.00%> (-100.00%)	:arrow_down:
src/label_to_subgroup.c	0.00% <0.00%> (-97.27%)	:arrow_down:
src/find_allowable_subgroup.c	0.00% <0.00%> (-97.15%)	:arrow_down:
src/central_auts.c	0.00% <0.00%> (-97.11%)	:arrow_down:
... and 58 more