ld wrapper improvements

[Ludovic Courtès]

With commit dacd5d2, the ‘validate-runpath’ phase reports RUNPATH
entries that do not point in the store.  This has allowed me to notice
that our ld wrapper has been adding RUNPATH entries pointing to the
build tree, sometimes using relative file names (see
<http://hydra.gnu.org/build/366062/nixlog/1/tail-reload> for instance.)

So with commit 51d0cd9b, the ld wrapper no longer adds -rpath arguments
for libraries that are not in the store.


Somewhat unrelated but commit c8bfa5b renames it to ld-wrapper.in, such
that commit 4ae7559f, which warns about package modules that cannot be
loaded, does not warn about this file.

Ludo’.