Honor additional LDFLAGS when exporting them on the commandline.

Change-Id: I794f8d9d23234af46f16a71021be069ca5383f03
Reviewed-on: https://gerrit.libreoffice.org/13568
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>