Rather than define ld_library_path_ver with a big nested $(if), just
put the overriding values in the makefiles for the relevant ports.

Also add a variable for port makefiles to append their own stuff to
with_temp_install, and use it to set LD_LIBRARY_PATH_RPATH=1 on
FreeBSD which is needed to make LD_LIBRARY_PATH override DT_RPATH
if DT_RUNPATH is not set (which seems to depend in unpredictable ways
on the choice of compiler, at least on my system).

Backpatch for the benefit of anyone doing regression tests on FreeBSD.
(For other platforms there should be no functional change.)