package/mesa3d: add llvm-config to meson [binaries]

package/mesa3d: add llvm-config to meson [binaries]

The meson buildsystem does not honour environment variables; instead, it
insists that path to programs be passed in the [binaries] section of the
cross-compilation.conf.

So, that is what we must do to pass the path to llvm-config.

Note that, LLVM_CONFIG does exist in the mesa3d source code, but it is
limited to the Scons buildsystem, and is also a leftover from when
mesa3d was using the autotools. It has never worked with the meson
buildsystem.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@gmail.com>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Tested-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>