build-sys: add a "pixman" feature

For now, pixman is mandatory, but we set config_host.h and Kconfig.
Once compilation is fixed, "pixman" will become actually optional.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
5 files changed