Fix CMake build rule for resource installation (#2687)

## Issue

The post build step for MaterialXTest does no trigger so resources are not copied to the
build area on a build. 

This means developers must manually copy this over when they are changing any files affecting the resources folder. If the lack of copy is not noticed / done then it can get confusing as to why source changes are not reflected in the build tests. 

## Change

Add an explicit target for resource copy and make MaterialXTest dependeng on it.

Author: kwokcb

source/MaterialXTest/CMakeLists.txt

@@ -110,13 +110,22 @@ if(MATERIALX_BUILD_GEN_MDL)
         DESTINATION ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/${MATERIALX_INSTALL_STDLIB_PATH}/mdl)
 endif()
 
-add_custom_command(TARGET MaterialXTest POST_BUILD
-    COMMAND ${CMAKE_COMMAND} -E copy_directory
-    ${CMAKE_CURRENT_SOURCE_DIR}/../../resources ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/resources)
+# Copy resources to build directory - always runs during build
+add_custom_target(MaterialXTestResources ALL
+    COMMAND ${CMAKE_COMMAND} -E copy_directory_if_different
+    ${CMAKE_CURRENT_SOURCE_DIR}/../../resources ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/resources
+    COMMENT "Copying resources to build directory")
+
+# Copy OSL utilities to build directory - always runs during build
+if(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/MaterialXRenderOsl/Utilities")
+    add_custom_target(MaterialXTestOslUtilities ALL
+        COMMAND ${CMAKE_COMMAND} -E copy_directory_if_different
+        ${CMAKE_CURRENT_SOURCE_DIR}/MaterialXRenderOsl/Utilities ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/resources/Utilities
+        COMMENT "Copying OSL utilities to build directory")
+    add_dependencies(MaterialXTestResources MaterialXTestOslUtilities)
+endif()
 
-add_custom_command(TARGET MaterialXTest POST_BUILD
-    COMMAND ${CMAKE_COMMAND} -E copy_directory
-    ${CMAKE_CURRENT_SOURCE_DIR}/MaterialXRenderOsl/Utilities ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/resources/Utilities)
+add_dependencies(MaterialXTest MaterialXTestResources)
 
 set_target_properties(
     MaterialXTest PROPERTIES