sdl: Fix compilation with libX11 >= 1.5.99.902

Applying an upstream patch to fix a compilation error with versions of
libX11 since 1.5.99.902. This patch modifies configure.in so we need to
run autogen.sh to make it working correctly. Running autoreconf will
fail because SDL uses bit tricky automake/autoconf configuration.

Upstream patch URL:
   http://hg.libsdl.org/SDL/rev/91ad7b43317a

Fixes:
   http://autobuild.buildroot.net/results/bf8/bf83466a7bbb0635d97fe279a18778d92f20bea2/



[Peter: add comment why we're doing this, add needed dependencies]
Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>