enable-x11 is set to yes by default and during configuration it has to
check for X11 header files on the path then the build system is prepared
to avoid compiling X11 related code if headers are not present.
Fixes #48
TEST="autogen.sh --prefix=<some prefix>, make sure X11 headers are not installed"
TEST="x11 should not print out as supported on the configure summary"
Signed-off-by: Daniel Charles <daniel.charles@intel.com>