session-env: Set correct libname on Debian systems.

Contributed by Jonah Brüchert.

Debian and its derivatives use paths like /usr/lib/$abi-triple, and
these /don't/ seem to also exist on non-Debian distros. So assume that
if a GNU ABI triple path is present that it should be used.

Differential Revision: https://phabricator.kde.org/D19864