libsvg-cairo: explicitly disable X support

Our libsvg-cairo package doesn't have any dependency on X, and the
source code doesn't seem to be using X. However, the configure script
checks for X headers and libraries. If they are found on the host,
/usr/lib and /usr/include get added to the library/header paths
causing weird build failures. This commit should fix this problem by
explicitly disabling the X support.

Should fix:

  http://autobuild.buildroot.org/results/d0f/d0fc233bb97c2ea18344746a5b7c63c8de65330a/
  http://autobuild.buildroot.org/results/0b3/0b3fb2ee9916aa265df5b6e4f04f6dfa25d1fc84/



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