targets/opencl: Fix (static) linking with LLVM (v2)

targets/opencl: Fix (static) linking with LLVM (v2)

Without this, I get linking failures (static linking).

The static linking is sort of required for me, because otherwise Steam and
applications using the Steam runtime regularily fail because my LLVM was
compiled and linked against a newer libgcc_s, libstdc++, etc. and uses
features from those newer versions. And instead of Steam just not
starting, my X starts crashing, whenever libGL fails to load a (32 bit)
driver.

Since I hate crashes of X and I don't think Valve/Steam will behave like
a proper distribution soon (rebuilds versus current Debian Testing, since
they base their Steam OS off that), I need a radeonsi which carries its
own LLVM within and doesn't care about what the runtime sets. This means
linking Mesa statically.

v1 → v2: Move logic to configure.ac

Acked-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Kai Wasserbäch <kai@dev.carbon-project.org>