Reject broken symlinks or even all

[fingolfin]

See gap-system/gap#5285

[fingolfin]

Perhaps we should just forbid all symlinks; that's easier to test for, and matches what the package distro does, see gap-system/PackageDistro@ae62ac6

[fingolfin]

Note that anyone installing GAP via Debian and then using that to make a GAP package release will likely get a bad symlink doc/mathjax due to this patch to GAPDoc applied in the Debian package... I didn't verify it but it seems likely that this also affects the Ubuntu package and any other package based on the Debian one.

This is quite unfortunate. We can't just fix this by removing the doc/mathjax file either, also the HTML files need to be adjusted to not reference it (basically undoing the changes introduced by the referenced patch.

Someone should also email Bill Allombert, the maintainer of the package, and kindly ask him whether he might undo that patch.