bug fix reported by Sergey Kosukhin for cc_opts handling

m4/ax_prog_cc_mpi.m4

@@ -172,9 +172,10 @@ AC_DEFUN_ONCE([_AX_PROG_CC_MPI], [
       AC_MSG_WARN([using CC=$CC as MPICC compiler.])
       MPICC="$CC"
     elif test -n "$CC" && test -n "$MPICC"; then
-      AC_MSG_WARN([overwriting CC=$MPICC. was former '$CC'])
       dnl extract found options from CC if overriden by MPICC
-      cc_opts=${CC##* }
+      cc_opts="$CC "
+      cc_opts=${cc_opts#* }
+      AC_MSG_WARN([overwriting CC='$MPICC $cc_opts'. was CC='$CC' ; MPICC='$MPICC'])
       CC="$MPICC $cc_opts"
     fi
   fi