BlueVK: optionally consult environment variable.
BlueVK: optionally consult environment variable.

One of Filament's Linux clients at Google wishes to specify a special
path to the SwiftShader library that proffers Vulkan entry points, and
this path can only be determined at run time.

An environment variable seems like the easiest way to support this
functionality. To prevent a security hazard, the env var is checked only
if a special build-time flag is enabled.

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