configure.ac: Move llvm_set_environment_variables higher.

configure.ac: Move llvm_set_environment_variables higher.

This moves the function to get the LLVM environment variables higher
in the file. It still needs to be below the "--enable-opencl" because
it uses $enable_opencl.
It can be called without condition now as it only throws errors if
openCL is enabled.

v5:
HAVE_MESA_LLVM is only used for gallium. Rename it to HAVE_GALLIUM_LLVM.
In order to only link LLVM when it is needed, HAVE_GALLIUM_LLVM is only
set if "$enable-gallium-llvm" is yes.

Signed-off-by: Tobias Droste <tdroste@gmx.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>