We currently always warn if SASL is not enabled. Ideally, we should only do so if it enabled but cannot be used due to missing libs/headers. This will not have any effect for the default use case, since SASL is enabled by default, but should reduce some noise when building the driver with SASL disabled.

This will be consistent with our plans for FLE in PHPC-1496, where we will only warn if FLE is enabled but not available due to crypto not being available.