Currently dmabuf_dep is found when the following conditions are met:
- linux/dma-buf.h is present;
- libdrm is found.
This is because Linux dmabuf support requires drm_fourcc.h which is part
of libdrm.
However, dmabuf_dep is used for two purposes:
- define HAVE_DMABUF to state dmabuf support;
- ensure the presence of drm_fourcc.h for gdk and for the
media-gstreamer module.
Decouple this, unconditionally check for libdrm and require it on
Linux. Then, use libdrm_dep only to state the drm_fourcc.h presence.
Given that now we unconditionally require libdrm on Linux, HAVE_DMABUF
depends only on the linux/dma-buf.h presence.
That way, we can work with older libdrm versions.
The list was generated via a bit of sed and grep from the current
dmabuf-fourcc.h, which is why I put it into its own file and included
all the formats, no matter how old they are.
