The goal is to avoid build breakage by pkg-config or whatever helpfully
putting default paths like -L/usr/lib64 into *_LIBS, which is entirely
useless since ld searches there anyway but may override other -L that
occur later on the command line for LO bundled externals.

On a Fedora 22 system, at least these variales were affected:
 CLUCENE_LIBS FIREBIRD_LIBS KDE4_LIBS POSTGRESQL_LIB BOOST_LDFLAGS

Change-Id: Ie55f65c3ae29a125f16871d95ad8b716abf5c982
Reviewed-on: https://gerrit.libreoffice.org/19784Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>