CMake: Compatibility fixes for CMake 3.4.0

CMake 3.4.0 no longer exports MPI_<LANGUAGE>_COMPILER as cached variable.
Therefore, we also have to make sure to unset ordinary variables (in
addition to cached variables) for our "retry" pass.

diff --git a/cmake/modules/FindMPI.cmake b/cmake/modules/FindMPI.cmake
index 1a8d0be9df7d000097e7e7ce1ae6d7e12dcd0793..4e45a53bd817c65277e505a2a0348f5caf38a2c8 100644 (file)
--- a/cmake/modules/FindMPI.cmake
+++ b/cmake/modules/FindMPI.cmake
@@ -80,6 +80,9 @@ IF(NOT MPI_CXX_FOUND AND DEAL_II_WITH_MPI)
     )
   # Clear variables so that FIND_PACKAGE runs again:
   SET(MPI_FOUND)
+  SET(MPI_CXX_COMPILER)
+  SET(MPI_C_COMPILER)
+  SET(MPI_Fortran_COMPILER)
   UNSET(MPI_CXX_COMPILER CACHE)
   UNSET(MPI_C_COMPILER CACHE)
   UNSET(MPI_Fortran_COMPILER CACHE)