Export MATERIALX_INSTALL_STDLIB_PATH in CMake config (#2724)

MATERIALX_INSTALL_STDLIB_PATH can be used to control where the data library is installed, but this needs to be reflected in the exported cmake config file as well.

@jfpanisset reported this in this [slack conversation](https://academysoftwarefdn.slack.com/archives/C0230LWBE2X/p1765194306347839).

Author: ld-kerley

cmake/modules/MaterialXConfig.cmake.in

@@ -20,7 +20,7 @@ include("${CMAKE_CURRENT_LIST_DIR}/@[email protected]")
 # MATERIALX_RESOURCES_DIR    Path to MaterialX Resources (sample data, mtlx etc)
 
 set_and_check(MATERIALX_BASE_DIR "@PACKAGE_CMAKE_INSTALL_PREFIX@")
-set_and_check(MATERIALX_STDLIB_DIR "@PACKAGE_CMAKE_INSTALL_PREFIX@/libraries")
+set_and_check(MATERIALX_STDLIB_DIR "@PACKAGE_CMAKE_INSTALL_PREFIX@/@MATERIALX_INSTALL_STDLIB_PATH@")
 if(@MATERIALX_BUILD_PYTHON@ AND @MATERIALX_INSTALL_PYTHON@)
     set_and_check(MATERIALX_PYTHON_DIR "@PACKAGE_CMAKE_INSTALL_PREFIX@/python")
 endif()