- if FOAM_EXT_LIBBIN is unset and some scripts set this to /usr/lib*
as a fallback (eg, to avoid an undefined value) this will cause a
system library to be found before appropriate *_ARCH_PATH entry.
This was noticed during a scotch compilation without third-party:
resulting in the system library (/usr/lib64/libscotch.so) to be found
instead of the SCOTCH_ARCH_PATH location
(/usr/lib64/mpi/gcc/openmpi/lib64/).
Simply changing the search order doesn't work for use, since we wish
to retain a preference for any dynamic libraries discovered in a
real FOAM_EXT_LIBBIN.
Circumvent these issues by only taking libraries from
FOAM_EXT_LIBBIN if it also points to a location within ThirdParty.
