Use of genreator expression to define

LIBRARY_OUTPUT_DIR since swig_add_library does not support
multi-config option.

tst/dtk-core-python/CMakeLists.txt

@@ -53,7 +53,7 @@ set(CMAKE_SWIG_FLAGS ${CMAKE_SWIG_PYTHON_FLAGS})
 swig_add_library(${SWIG_MODULE_NAME}
   TYPE MODULE
   LANGUAGE PYTHON
-  OUTPUT_DIR ${PYTHON_BUILD_PATH}/dtk
+  OUTPUT_DIR $<PATH:APPEND,${PYTHON_BUILD_PATH},dtk>
   SOURCES Dummy.i)
 
 target_link_libraries(${SWIG_MODULE_NAME} PRIVATE dtk::Core)
@@ -69,7 +69,7 @@ target_link_libraries(${SWIG_MODULE_NAME} PUBLIC Python3::Python)
 set_target_properties(${SWIG_MODULE_NAME} PROPERTIES
   SWIG_USE_TARGET_INCLUDE_DIRECTORIES ON
   OUTPUT_NAME ${SWIG_MODULE_NAME}
-  LIBRARY_OUTPUT_DIRECTORY ${PYTHON_BUILD_PATH}/dtk)
+  LIBRARY_OUTPUT_DIRECTORY $<PATH:APPEND,${PYTHON_BUILD_PATH},dtk>)
 
 ## ###################################################################
 ## Test build rules

wrp/dtkCore/CMakeLists.txt

@@ -25,7 +25,7 @@ set(CMAKE_SWIG_FLAGS ${CMAKE_SWIG_PYTHON_FLAGS})
 swig_add_library(${SWIG_MODULE_NAME}
   TYPE MODULE
   LANGUAGE PYTHON
-  OUTPUT_DIR ${PYTHON_BUILD_PATH}/dtk
+  OUTPUT_DIR $<PATH:APPEND,${PYTHON_BUILD_PATH},dtk>
   SOURCES ${LAYER_NAME}.i)
 
 target_link_libraries(${SWIG_MODULE_NAME} PRIVATE dtk::Core)
@@ -39,7 +39,7 @@ target_link_libraries(${SWIG_MODULE_NAME} PUBLIC Python3::Python)
 set_target_properties(${SWIG_MODULE_NAME} PROPERTIES
   SWIG_USE_TARGET_INCLUDE_DIRECTORIES ON
   OUTPUT_NAME ${SWIG_MODULE_NAME}
-  LIBRARY_OUTPUT_DIRECTORY ${PYTHON_BUILD_PATH}/dtk)
+  LIBRARY_OUTPUT_DIRECTORY $<PATH:APPEND,${PYTHON_BUILD_PATH},dtk>)
 
 ## #############################################################################
 ## Install rules