mesa3d: specify llvm-prefix

When it's not defined an automatic search is triggered, which can find
/usr/bin/llvm-config on distributions that include all development files
(like gentoo) and have desktop packages installed.
So point it towards the staging dir which is where it should live
eventually if/when we've got it to avoid header/include path polution
that otherwise leads to build failures.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>