Save full path of MPI launcher.

* This reduces conflicts arising from later changes of PATH at build
  vs configure time.

configure

@@ -26594,20 +26594,22 @@ do
 set dummy $ac_prog; ac_word=$2
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
 $as_echo_n "checking for $ac_word... " >&6; }
-if ${ac_cv_prog_MPI_LAUNCH+:} false; then :
+if ${ac_cv_path_MPI_LAUNCH+:} false; then :
   $as_echo_n "(cached) " >&6
 else
-  if test -n "$MPI_LAUNCH"; then
-  ac_cv_prog_MPI_LAUNCH="$MPI_LAUNCH" # Let the user override the test.
-else
-as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+  case $MPI_LAUNCH in
+  [\\/]* | ?:[\\/]*)
+  ac_cv_path_MPI_LAUNCH="$MPI_LAUNCH" # Let the user override the test with a path.
+  ;;
+  *)
+  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
 for as_dir in $PATH
 do
   IFS=$as_save_IFS
   test -z "$as_dir" && as_dir=.
     for ac_exec_ext in '' $ac_executable_extensions; do
   if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
-    ac_cv_prog_MPI_LAUNCH="$ac_prog"
+    ac_cv_path_MPI_LAUNCH="$as_dir/$ac_word$ac_exec_ext"
     $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
     break 2
   fi
@@ -26615,9 +26617,10 @@ done
   done
 IFS=$as_save_IFS
+  ;;
+esac
 fi
-fi
-MPI_LAUNCH=$ac_cv_prog_MPI_LAUNCH
+MPI_LAUNCH=$ac_cv_path_MPI_LAUNCH
 if test -n "$MPI_LAUNCH"; then
   { $as_echo "$as_me:${as_lineno-$LINENO}: result: $MPI_LAUNCH" >&5
 $as_echo "$MPI_LAUNCH" >&6; }

configure.ac

@@ -94,7 +94,7 @@ AS_IF([test x"${enable_mpi}" = x"yes"],
 HAVE_PARALLEL_NC4=0
 AS_IF([test x"$USE_MPI" = xyes],
   [AC_DEFINE([USE_MPI],[1],[parallel I/O requested and available])
-   AC_CHECK_PROGS([MPI_LAUNCH],[mpirun mpiexec],[])
+   AC_PATH_PROGS([MPI_LAUNCH],[mpirun mpiexec],[])
    AS_IF([test x$MPI_LAUNCH = x],[])
    AS_IF([test x"$ENABLE_NC4" = xyes],
      [AC_CHECK_HEADERS([netcdf_par.h],