MATX-344 : Ensure CMake config relocatability

[JGamache-autodesk]

All install paths must be relative for CMake to generate a relocatable
config file. This means removing all those CMAKE_INSTALL_PREFIX
path components that point to an absolute path.

For an install in /dev/MaterialX/staging
The difference will be in the generated file:
/dev/MaterialX/staging/lib/cmake/MaterialX/MaterialXTargets.cmake
Where, without this fix, you will find:

set(_IMPORT_PREFIX "/dev/MaterialX/staging")

and, with the fix, the results will be:

get_filename_component(_IMPORT_PREFIX "${CMAKE_CURRENT_LIST_FILE}" PATH)
get_filename_component(_IMPORT_PREFIX "${_IMPORT_PREFIX}" PATH)
get_filename_component(_IMPORT_PREFIX "${_IMPORT_PREFIX}" PATH)
get_filename_component(_IMPORT_PREFIX "${_IMPORT_PREFIX}" PATH)
if(_IMPORT_PREFIX STREQUAL "/")
  set(_IMPORT_PREFIX "")
endif()

which makes the install area packageable and relocatable.

[kwokcb]

Is this correct in understanding the change:

1. CMAKE_INSTALL_PREFIX is automatically prepended so all installs should be relative to whatever this is set to.
2. If the default location is set to this, we still override the location to be the install location (relative to where the build occurs).
3. The appropriate expansion is performed when creating the MaterialXTargets.cmake file.
   Thanks.

[kwokcb]

LGTM. But want to check my understanding.

[JGamache-autodesk]

@kwokcb you are correct on all 3 points. Setting the default location in CMake is done by adding

-DCMAKE_INSTALL_PREFIX=C:\Ws\staging

to the command line, so basically the same variable name.