scons: define OPTIONAL_LIBSHEADERS before use
scons: define OPTIONAL_LIBSHEADERS before use

Refs #4075

(cherry picked from commit 0d68464194bc32fad3f6b55eac17b31ba60ea1da)

naturalatlas/mapnik,naturalatlas/mapnik,lightmare/mapnik,tomhughes/mapnik,tomhughes/mapnik,tomhughes/mapnik,mapnik/mapnik,tomhughes/mapnik,naturalatlas/mapnik,mapnik/mapnik,lightmare/mapnik,mapnik/mapnik,naturalatlas/mapnik,mapnik/mapnik,lightmare/mapnik,lightmare/mapnik