There was a request to move the lib folder off the SSD the blender code was on due to the size of the libs, which seemed like a reasonable thing to support.
This diff adds the option by setting the BLENDER_LIB_DIR environment variable. Initially i thought that adding the option to make.bat would suffice but you would end up adding libdir d:\the\new\place\the\libs\live\ to every make.bat invocation which is... "less than ideal" an environment var is the way the go here.
The BLENDER_LIB_DIR needs to be set to the root of the library directory (so should *not* include the win64_vc15 bit)
I'm pretty sure i didn't break anything for other platforms except for the change in tests/python/CMakeLists.txt tested on windows, I think it'll work for the other platforms but a second pair of eyes wouldn't hurt there.