[Feature Request] external libraries support.

[bartoszek]

I've recently encountered a problem with bundled openvdb relying on deprecated tbb headers. Any chance for allowing building against external libs.

Suggested libs to "externalize" :

• openvdb
• opensubdiv
• opencolorio
• expat
• yamlcpp

Header only libraries:

• eigen
• nohlman-json
• spdlog

I've concocted an example for openvdb, woks for luxcorerender_v2.5 against openvdg:9.0.0

Resolves #572

[acasta69]

The build failed, apparently only for Windows but really for all platforms. I think the reason is that an external OpenVDB is not available here.
My opinion is that this could be accepted, but I would suggest to reverse the logic, so that if we do nothing, i.e. no additional symbol is defined, the build is done with the bundled OpenVDB. If someone has a separate OpenVDB, they can define the symbol to have it used.
Of course we can only support the bundled version if something does not work as expected.