...after ecd5d49d "liblangtag: prevent libtool
from adding annoying stuff to RPATH".  "Outsmarting" libtool in the way
described in 1d028d47 "Fix Linux RPATH of
various external modules" only works if there's no utility applications in the
outsmarted module, that are run while building the outsmarted module (cf. the
redland case in 1d028d47).  To work around that,
pass in an LD_LIBRARY_PATH (or equivalent) that finds the relevant libs as built
in that module.

(And gb_Helper_extend_ld_path was until now only available in com_GCC_defs.mk.
There, it accepts any number of paths separated by ":".  When used in a
platform-independend way like it is used here, it will only support a single
path.)

Change-Id: Ic615e436b19b8b9aa7daa019c8f5287de90c3abc