ENH: Use the HINTS feature of find_library to find the right libraries for

  MPI, and act a bit more intelligently when MPI cannot be found.

Modules/FindMPI.cmake

@@ -210,11 +210,11 @@ elseif (MPI_COMPILE_CMDLINE)
   foreach(LIB ${MPI_LIBNAMES})
     string(REGEX REPLACE "^-l" "" LIB ${LIB})
     set(MPI_LIB "MPI_LIB-NOTFOUND" CACHE FILEPATH "Cleared" FORCE)
-    find_library(MPI_LIB ${LIB} PATHS ${MPI_LINK_PATH})
+    find_library(MPI_LIB ${LIB} HINTS ${MPI_LINK_PATH})
     if (MPI_LIB)
       list(APPEND MPI_LIBRARIES ${MPI_LIB})
     else (MPI_LIB)
-      status(ERROR "Unable to find MPI library ${LIB}")
+      message(SEND_ERROR "Unable to find MPI library ${LIB}")
     endif (MPI_LIB)
   endforeach(LIB)
   set(MPI_LIB "MPI_LIB-NOTFOUND" CACHE INTERNAL "Scratch variable for MPI detection" FORCE)
@@ -222,12 +222,13 @@ elseif (MPI_COMPILE_CMDLINE)
   # Chop MPI_LIBRARIES into the old-style MPI_LIBRARY and
   # MPI_EXTRA_LIBRARY.
   list(LENGTH MPI_LIBRARIES MPI_NUMLIBS)
-  if (MPI_NUMLIBS GREATER 0)
+  list(LENGTH MPI_LIBNAMES MPI_NUMLIBS_EXPECTED)
+  if (MPI_NUMLIBS EQUAL MPI_NUMLIBS_EXPECTED)
     list(GET MPI_LIBRARIES 0 MPI_LIBRARY_WORK)
     set(MPI_LIBRARY ${MPI_LIBRARY_WORK} CACHE FILEPATH "MPI library to link against" FORCE)
-  else (MPI_NUMLIBS GREATER 0)
-    set(MPI_LIBRARY "MPI_LIBRARY-NOTFOUND" CACHE STRING "MPI library to link against" FORCE)
-  endif (MPI_NUMLIBS GREATER 0)
+  else (MPI_NUMLIBS EQUAL MPI_NUMLIBS_EXPECTED)
+    set(MPI_LIBRARY "MPI_LIBRARY-NOTFOUND" CACHE FILEPATH "MPI library to link against" FORCE)
+  endif (MPI_NUMLIBS EQUAL MPI_NUMLIBS_EXPECTED)
   if (MPI_NUMLIBS GREATER 1)
     set(MPI_EXTRA_LIBRARY_WORK ${MPI_LIBRARIES})
     list(REMOVE_AT MPI_EXTRA_LIBRARY_WORK 0)