The current way "use_subzero" was used seems to fail when
compiling with goma (I don't see why, honestly). Anyway,
most build configuration related booleans are declared in
a declare_args() section, so I moved the setting there,
which will hopefully solve the issue.

Change-Id: I063d9938d04bfdf8140878a740f3710309f8bf1e
Reviewed-on: https://swiftshader-review.googlesource.com/8714Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>