qdecoder: autoreconf and fix c99 detection

Fixes http://autobuild.buildroot.net/results/da8/da8ce760cdabeb9d4cda00dcc801817e4b4eb12c/



As we're patching configure.ac we need to autoreconf the package.
After reconf, the c99 check fails, so force it like we do elsewhere.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>