mozjs: add PACKAGECONFIG for x11

Add PACKAGECONFIG for x11 and enable it when x11 is
included in DISTRO_FEATURES, and specify the correct
path for x-includes and x-libraries to avoid
searching the host libraries.

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>