Merge branch 'fix-findmpi-version-check' into release

* fix-findmpi-version-check:
  Fix backwards compatibility logic for FindMPI
  Bump FindMPI to v3.15.1, modified for v3.6 compatibility.
  Use a 4 part version string in between releases

cmake/FindMPI.cmake

@@ -4,7 +4,7 @@
 #------------------------------------------------------------------------------#
 
 # This module is already included in new versions of CMake
-if(CMAKE_VERSION VERSION_LESS 4.0)
+if(CMAKE_VERSION VERSION_LESS 3.15.1)
   include(${CMAKE_CURRENT_LIST_DIR}/upstream/FindMPI.cmake)
 else()
   include(${CMAKE_ROOT}/Modules/FindMPI.cmake)

cmake/upstream/FindMPI/libver_mpi.c

 #include <mpi.h>
 
 #ifdef __cplusplus
-#include <cstdio>
+#  include <cstdio>
 #else
-#include <stdio.h>
+#  include <stdio.h>
 #endif
 
 int main(int argc, char* argv[])
@@ -16,4 +16,5 @@ int main(int argc, char* argv[])
 #else
   puts(mpilibver_str);
 #endif
+  return 0;
 }

cmake/upstream/FindMPI/test_mpi.c

 #include <mpi.h>
 
 #ifdef __cplusplus
-#include <cstdio>
+#  include <cstdio>
 #else
-#include <stdio.h>
+#  include <stdio.h>
 #endif
 
 #if defined(MPI_VERSION) && defined(MPI_SUBVERSION)
@@ -21,11 +21,11 @@ const char mpiver_str[] = { 'I', 'N',
 int main(int argc, char* argv[])
 {
 #if defined(MPI_VERSION) && defined(MPI_SUBVERSION)
-#ifdef __cplusplus
+#  ifdef __cplusplus
   std::puts(mpiver_str);
-#else
+#  else
   puts(mpiver_str);
-#endif
+#  endif
 #endif
 #ifdef TEST_MPI_MPICXX
   MPI::MPI_Init(&argc, &argv);
@@ -34,4 +34,5 @@ int main(int argc, char* argv[])
   MPI_Init(&argc, &argv);
   MPI_Finalize();
 #endif
+  return 0;
 }