fix pythonpath by using generator expression

With multi-configuration/crossplatform toolchains, the output path of a target varies from the selected configuration (most famous: Debug/Release on MSVC/xcode). Therefore, the module's output path is not the bin dir, but mostly in something like ${CMAKE_CURRENT_BINARY_DIR}/Debug or ${CMAKE_CURRENT_BINARY_DIR}/Release.

This MR fixes the path, so that pythonpath is always set to the correct path where the module was built into.

CMakeLists.txt

@@ -39,5 +39,5 @@ add_test(
 )
 set_tests_properties(
 	discover_tests PROPERTIES
-	ENVIRONMENT PYTHONPATH=${CMAKE_CURRENT_BINARY_DIR}
+	ENVIRONMENT "PYTHONPATH=$<TARGET_FILE_DIR:openmesh>"
 )