mesa: allow to disable xmlconfig support

mesa: allow to disable xmlconfig support

>From my understanding, xmlconfig is useful for "dynamic" driconf
support, i.e. driconf the user specifies at runtime.

According to the wiki[1], driconf is useful for OpenGL drivers.

I wager we mostly don't need xmlconfig at all which may allow us to also
get rid (in most cases) of the expat dependency. But that is an
optimization to investigate later, so let's keep xmlconfig enabled for
now as it is the default whenever the meson feature is not disabled
(defaults to auto) and expat is found (currently part of DEPENDS in
mesa.inc).

This will be useful for mesa-tools-native which isn't meant to compile
drivers and thus shouldn't try to compile driconf support and install
driconf example files.

Technically, xmlconfig depends on the expat meson feature (and the
presence of the expat build dependency) but the feature is default auto
so having expat dependency in xmlconfig PACKAGECONFIG seems enough
instead of having one PACKAGECONFIG depend on another PACKAGECONFIG's
presence.

[1] https://dri.freedesktop.org/wiki/DriConf/

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Tested-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>