library paths: Explicitly add /usr/local/lib

Hardened runtime prevents the program to load any libraries outside
of the application bundle by default. Specifically, it also ignores
DYLD_FALLBACK_PATH which would contain /usr/local/lib.
To allow usage of a library outside of the application bundle, add
/usr/local/lib explicitly.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>