mesa: build xmlconfig to a separate static library

mesa: build xmlconfig to a separate static library

As we use the file from both the dri modules and loader, we end up with
multiple definition of the symbols provided in our gallium dri  modules.
Additionally we compile the file twice.

Resolve both issues, effectively enabling the build on toolchains which
don't support -Wl,--allow-multiple-definition.

v2: [Emil Velikov]
 - Fix the Scons/Android build.
 - Resolve libgbm build issues (bring back the missing -lm)

Cc: Julien Isorce <j.isorce@samsung.com>
Cc: "10.5 10.6" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90310
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90905
Acked-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>