Improve support for manuals using old-style gapmacros.tex, doc/make_doc script

[fingolfin]

Right now, this is a bit of a hack: Those make_doc scripts usually hardcode paths involving ../../... While that can be fixed in various ways, more problematic is that they usually reference the

Thus, running ./make_doc only works if the package either is really inside GAPROOT/pkg/PACKAGE, or else suitable symlinks (to GAPROOT/etc and GAPROOT/doc are created in the right place relative to the make_doc script.

One way to deal with that would be to wrap the package not directly in tmp, but rather in tmp/pkg/, and add the above mentioned symlinks as tmp/doc/ and tmp/etc

Another approach would be to make the ../../.. adjustable (this obviously requires modifying those packages):

• in make_doc, on could add at the top something like GAPDIR=${GAPDIR:-../../..} (or, for scripts which have a ./configure, the GAPROOT passed to configure could be inserted into the make_doc script)
• in the TeX code, perhaps the ../../.. could be moved into a tex variable, which can be overridden via an argument to tex. Or for projects with a ./configure, one could have a foo.tex.in file in which @GAPDIR@ is used and substituted appropriately. (But careful: we must make sure that in the final file, references to the GAP manuals are still relative, at least for the release tarball.