Add more support for configuring MPI

Handles options --enable-mpi and --with-mpi-prefix for compatibility
with other autotools projects which uses these settings to tell the
build system where the local MPI library is.

configure

@@ -166,6 +166,10 @@ for OPT in "$@"; do
               pch_use=" -DPRECOMPILE_HEADERS:BOOL=ON"
               rootvar=""
               ;;
+            mpi         |\
+            mpi-prefix)
+              rootvar="_MPI_PREFIX_PATH"
+              ;;
             agmg        |\
             ert         |\
             boost       |\
@@ -232,6 +236,7 @@ for OPT in "$@"; do
               use_openmp=" -DUSE_OPENMP=OFF"
               pkgname=""
               ;;
+            mpi  | \
             parallel)
               use_mpi=" -DUSE_MPI=OFF"
               pkgname=""
@@ -270,6 +275,7 @@ for OPT in "$@"; do
               # special flag; don't set shared/static
               shared=""
               ;;
+            mpi  | \
             parallel)
               use_openmp=" -DUSE_MPI=ON"
               # special flag; don't set shared/static