scripts: exporting (even empty) LIBRARY_PATH and/or LPATH breaks gcc

If either LIBRARY_PATH or LPATH is set, even to the empty string,
the gcc build breaks.

Fix that by bailing-out rather than re-setting.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 reporter_name="Bart vdr. Meulen <bartvdrmeulen@gmail.com>"

 reporter_url=""

 reporter_comment="Config to build cross-compiler with mingw as kernel type"