Omit Python data installation in non-Python builds (#1437)

Since the addition of #1237, we now create a python folder, with a copy of the stdlibs, even if we're not building python.

This is at best redundant, and in my particular case, caused a problem in a downstream build script.

libraries/CMakeLists.txt

@@ -18,9 +18,11 @@ if(SKBUILD)
     set(MATERIALX_PYTHON_LIBRARIES_PATH "${SKBUILD_PLATLIB_DIR}/MaterialX/libraries")
 endif()
 
-install(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/
-        DESTINATION "${MATERIALX_PYTHON_LIBRARIES_PATH}"
-        PATTERN "CMakeLists.txt" EXCLUDE
-        PATTERN "pbrlib_genosl_impl.*" EXCLUDE)
-install(FILES "${CMAKE_CURRENT_SOURCE_DIR}/pbrlib/genosl/pbrlib_genosl_impl.${PBRLIB_SUFFIX}"
-        DESTINATION "${MATERIALX_PYTHON_LIBRARIES_PATH}/pbrlib/genosl/" RENAME pbrlib_genosl_impl.mtlx)
+if(MATERIALX_BUILD_PYTHON)
+    install(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/
+            DESTINATION "${MATERIALX_PYTHON_LIBRARIES_PATH}"
+            PATTERN "CMakeLists.txt" EXCLUDE
+            PATTERN "pbrlib_genosl_impl.*" EXCLUDE)
+    install(FILES "${CMAKE_CURRENT_SOURCE_DIR}/pbrlib/genosl/pbrlib_genosl_impl.${PBRLIB_SUFFIX}"
+            DESTINATION "${MATERIALX_PYTHON_LIBRARIES_PATH}/pbrlib/genosl/" RENAME pbrlib_genosl_impl.mtlx)
+endif()