buildsys: always link gap against libgap

[fingolfin]

... even the copy we don't install system wide.

On macOS this allows us an annoying issue with the linker flags used by e.g. gac for building kernel extensions: by always linking them against libgap we don't need to use a flat namespace, nor rely on a bundle loader (which breaks if you try to load the kernel extension in another binary using libgap).

However, this is not yet working on Linux at all, as there it won't find the not-yet-installed libgap:

$ ./gap
./gap: error while loading shared libraries: libgap.so.9: cannot open shared object file: No such file or directory

In principle this can be dealt with but I don't have time to do that right now, but I also didn't want to let this fragment get lost, so this PR will remind me to finish this one day (if someone would like to help out, that'd be welcome, too).

Alternatively one could restrict this change to macOS.