package/mesa3d-headers: fix install of dri.pc

package/mesa3d-headers: fix install of dri.pc

mesa3d-headers is a little bit special: it uses the same sources as
the mesa3d package, named just 'mesa' upstream. mesa uses the meson
buildsystem, an that is what we use in the mesa3d package.

However, mesa3d-headers does not install the whole of mesa; it only
installs a select set of headers for those binary blobs that do not
provide them.

mesa does not provide such a feature (only installing headers) with
its meson buildsystem. As a consequence, we've made mesa3d-headers a
generic package, that basically only copies headers over.

Additionally, mesa3d-headers also provides the dri.pc file for when
Xorg is enabled; see 7468b60e7c (package/mesa3d-headers: also install
dri header and .pc file).

We used to manually generate that file from a .in template that was
present in mesa source code at the time it was still using autotools.
But when they switched over to using meson, the template was dropped
[0], and the dri.pc is now entirely generated using meson internals
[1].

So we now have no template present in the source code, so we must
come up with our own. This simplifies the replacement pattern to
just inject the version string.

[0] https://cgit.freedesktop.org/mesa/mesa/commit/?id=158758618264eac113025a86a360dc305ed4498b
[1] https://cgit.freedesktop.org/mesa/mesa/tree/src/mesa/drivers/dri/meson.build?h=19.2#n93

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Tested-by: Vincent Fazio <vfazio@xes-inc.com>
[yann.morin.1998@free.fr:
  - entirely rework the commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>