Overhaul build system

[fingolfin]

• configure now supports ./configure --with-gaproot=GAPROOT in addition to ./configure GAPROOT
• configure also performs a check whether the given GAPROOT directory seems like a valid gap root
• Makefile now has a rule to re-run configure (to regenerate itself) if Makefile.in or configure or GAP's sysinfo.gap changed; this is very convenient when editing the build system
• pass --enable-generic to nauty's configure script, to disable use of -march=native which has caused us trouble in the past
• Makefile: linearized the binaries target to make it a bit easier to see what's going on, esp. when editing part of it

This puts it in line with e.g. datastructures and other packages

[codecov[bot]]

Codecov Report

Merging #43 (5946287) into master (c1ae51c) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #43   +/-   ##
=======================================
  Coverage   79.90%   79.90%           
=======================================
  Files           2        2           
  Lines        3882     3882           
=======================================
  Hits         3102     3102           
  Misses        780      780