ffmpeg: fix configure failure on noexec /tmp host

ffmpeg: fix configure failure on noexec /tmp host

The configure scripts uses /tmp to execute some generated files.
If /tmp is noexec, then we meet the following error.

  | Unable to create and execute files in /tmp.  Set the TMPDIR environment
  | variable to another directory and make sure that it is not mounted noexec.
  | Sanity test failed.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>