FindMPI: Fix parsing of mpicc -Wl,-L link flags (#9093)

Commit d84cbd0f (FindMPI: Parse mpicc flags more carefully, 2010-06-24)
broke parsing of '-L' flags appearing after '-Wl,' by expecting a
preceding space.  Update the regular expression to allow '-Wl,-L' too.

Modules/FindMPI.cmake

@@ -230,10 +230,10 @@ elseif (MPI_COMPILE_CMDLINE)
   endif (NOT MPI_INCLUDE_PATH_WORK)
 
   # Extract linker paths from the link command line
-  string(REGEX MATCHALL "(^| )-L([^\" ]+|\"[^\"]+\")" MPI_ALL_LINK_PATHS "${MPI_LINK_CMDLINE}")
+  string(REGEX MATCHALL "(^| |-Wl,)-L([^\" ]+|\"[^\"]+\")" MPI_ALL_LINK_PATHS "${MPI_LINK_CMDLINE}")
   set(MPI_LINK_PATH)
   foreach(LPATH ${MPI_ALL_LINK_PATHS})
-    string(REGEX REPLACE "^ ?-L" "" LPATH ${LPATH})
+    string(REGEX REPLACE "^(| |-Wl,)-L" "" LPATH ${LPATH})
     string(REGEX REPLACE "//" "/" LPATH ${LPATH})
     list(APPEND MPI_LINK_PATH ${LPATH})
   endforeach(LPATH)