ENH: reduce reliance on FOAM_EXT_LIBBIN during builds
- just check WM_PROJECT_DIR instead. - provide a fallback value when FOAM_EXT_LIBBIN might actually be needed. Only strictly need FOAM_EXT_LIBBIN for scotch/metis decomposition, and when these are actually supplied by ThirdParty. All other ThirdParty dependencies are referenced by BOOST_ARCH_PATH etc. Can therefore drop the FOAM_EXT_LIBBIN dependency for VTK-related things, which do not use scotch/metis anyhow.