At least make-4.1-5.fc24.x86_64's /usr/bin/make (indirectly) links against
libfreebl3.so, so it could erroneously pick up our instdir/program/libfreebl3.so
delivered there from external/nss.  But that's a problem for ASan/UBSan builds,
where that libfreebl3.so is instrumented and expects to find certain symbols
exported from the executable (and which /usr/bin/make of course doesn't have),
so running make from within external/liblangtag/ExternalProject_langtag.mk
fails.

Turns out that the only place where LD_LIBRARY_PATH is needed during the build
of external/liblangtag is when running
workdir/UnpackedTarget/langtag/data/reg2xml.

(This is unrelated to the recent changes to external/liblangtag by the way; just
happend to show up now by accident, when doing an incremental build where
external/nss had already been built when external/liblangtag got rebuilt.
external/firebird has a similar problem, but everybody seems to run ASan/UBSan
builds with --disable-firebird-sdbc anyway for now.)

Change-Id: I6e045b6d33a154e350f4640265e6568f96634187
Reviewed-on: https://gerrit.libreoffice.org/29211Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>