Makefile.gappkg improvements

[fingolfin]

Just to make sure I won't forget about these:

• [x] add support for user-supplied CFLAGS, CXXFLAGS, LDFLAGS (this may fix coverage reporting via Travis&Codecov for packages using it)
• [ ] add a version number into Makefile.gappkg which configure scripts can use to decide if the copy of Makefile.gappkg in GAP is actually newer than the one bundled with the package (to this end, it should be a simple integer that is easy to extract with grep or awk or so)
• [x] some packages combine Makefile.gappkg with autoconf; this requires some extra code right now in their Makefile.in files; think about whether we can somehow move this into Makefile.gappkg in a generic way
• [ ] think about ways to add support for make install support (this is a big one)
• [ ] update all packages that use Makefile.gappkg to the latest and greatest

[fingolfin]

I actually implemented the second point (adding a version number & automatic selfupdate), but I think I won't submit it as a PR: The more I think about it, the more I believe it is better to do this manually. The one point going for the autoupdate is that it'll "happen" to package maintainers, who those perhaps will be urged (annoyed?) to update to new versions (otherwise the release-gap-package script won't run through). While that seems appealing on the surface (right now I need to prod package authors / submit lots of PRs to get an update out), I don't think it warrants the efforts, complexity, and possible source of serious annoyance (like, if a new version of the file is broken for some package, then that package shouldn't have to switch to it). And in general, it's OK to keep using an older version of it; if a new version adds compelling new features (e.g. make install support), that should be enough to either convince package authors to update by themselves, or us to submit PRs to them to get them to update.