Wrapped in a `if(NOT DEFINED SWIFTSHADER_LOGGING_LEVEL)` so that it can be controlled by a parent project with a simple `set()`, much like `option_if_not_defined()`.

Bug: b/169966461
Change-Id: I53a072289e6eac589e7640970045174076492774
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48948
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>