gtkmm24 fails to link because libGL.so.1 not found

[scratch65535 at att.net]

Another case where the makefile is at fault.   The only place
there's a libGL.so.1 as such is in /compat/linux, but I wasn't
able to modify the makefile successfully to point at that
subtree.

First I tried an additional -L switch, and then an -R.  Neither
worked.  If I should have used something else, I hope someone
will clue me up because they're the only two I know for the ld
loader..