janus-gateway: add a patch to use -Wunused-but-set-variable only when available

Fixes:

  http://autobuild.buildroot.org/results/3f0/3f07574e6e4edda9e31fcb0de520a4dbabe6b94a/



[Thomas:
 - Improved configure.ac logic, as suggested by Yann E. Morin.
 - Added a comment in the .mk file to indicate why we're using
   AUTORECONF = YES. Suggested by Yann as well.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>