MaterialX install is not relocatable

[JGamache-autodesk]

This means a MaterialX install done to a staging area using -DCMAKE_INSTALL_PREFIX=/my/stage can not be packaged and installed somewhere else.

Problem is in /my/stage/lib/cmake/MaterialX/MaterialXTargets.cmake which will contain the line:

set(_IMPORT_PREFIX "/my/stage")

There is a mechanism in CMake to keep the MaterialX generated CMake file relocatable and it hinges on keeping all install paths relative.

I would recommend centralizing the library creation into a CMake function that would insure all path are local, this is what is done in USD and it generates a relocatable CMake file.

[JGamache-autodesk]

Fixed via PR #1350 in this repo.
Merged back to ILM repo via AcademySoftwareFoundation#805