Do not use both TARGET and DEPENDS in ADD_CUSTOM_COMMAND.

Modern cmake warns like this:

CMake Warning (dev) at po/CMakeLists.txt:20 (ADD_CUSTOM_COMMAND):
  The following keywords are not supported when using
  add_custom_command(TARGET): DEPENDS.

Rewrite the build instructions to have an additional level of dependencies.
Before, we have one build target, the compiles all *.po files. Now we let
the main target ("translations") depend on the output files (*.gmo), which
themselves depend on their respective *.po file via ADD_CUSTOM_COMMAND.

I took the example CMakeLists.txt at Techbase[1] as inspiration how to
rewrite the target chain.

[1] https://techbase.kde.org/Development/Tutorials/Localization/i18n_Build_Systems/Outside_KDE_repositories

po/CMakeLists.txt

@@ -9,21 +9,23 @@ ELSE(NOT GETTEXT_MSGFMT_EXECUTABLE)
 
 	SET(catalogname kdbg)
 
-	ADD_CUSTOM_TARGET(translations ALL)
-
 	FILE(GLOB PO_FILES *.po)
+	SET(GMO_FILES)
 
 	FOREACH(_poFile ${PO_FILES})
 		GET_FILENAME_COMPONENT(_lang ${_poFile} NAME_WE)
 		SET(_gmoFile ${CMAKE_CURRENT_BINARY_DIR}/${_lang}.gmo)
 
-		ADD_CUSTOM_COMMAND(TARGET translations
+		ADD_CUSTOM_COMMAND(OUTPUT ${_gmoFile}
 			COMMAND ${GETTEXT_MSGFMT_EXECUTABLE} --check -o ${_gmoFile} ${_poFile}
 			DEPENDS ${_poFile})
 		INSTALL(FILES ${_gmoFile}
 			DESTINATION ${LOCALE_INSTALL_DIR}/${_lang}/LC_MESSAGES/
 			RENAME ${catalogname}.mo)
+		LIST(APPEND GMO_FILES ${_gmoFile})
 
 	ENDFOREACH(_poFile ${PO_FILES})
 
+	ADD_CUSTOM_TARGET(translations ALL DEPENDS ${GMO_FILES})
+
 ENDIF(NOT GETTEXT_MSGFMT_EXECUTABLE)