Many systems have TEMPDIR mounted noexec, warn about this.

Originally committed as revision 6833 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1137,7 +1137,8 @@ chmod +x $TMPE >>$logfile 2>&1
 if ! $TMPE >>$logfile 2>&1; then
     cat <<EOF
 Unable to create and execute files in $TMPDIR1.  Set the TMPDIR environment
-variable to another directory.
+variable to another directory and make sure that $TMPDIR1 is not mounted
+noexec.
 EOF
     die "Sanity test failed."
 fi