ENH: additional handling of questionable bad FOAM_EXT_LIBBIN values

- 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.

wmake/scripts/have_adios2

@@ -79,9 +79,8 @@ have_adios2()
     then
         header=$(findFirstFile $ADIOS2_ARCH_PATH/include/$header)
 
-        # FOAM_EXT_LIBBIN is allowed to be unset
         library=$(findFirstFile \
-            $FOAM_EXT_LIBBIN/$library \
+            "$(thirdExtLib $library)" \
             $ADIOS2_ARCH_PATH/lib/$library \
             $ADIOS2_ARCH_PATH/lib$WM_COMPILER_LIB_ARCH/$library \
         )