CMake: avoid unnecessarily remaking all .mo files on each build

All the .mo files had a dependency on the target directory, which meant that
every change to the timestamp of that directory caused all the files to be
remade; and each file remade caused a change to the directory's timestamp ...

Since CMake 3.0, "cmake -E make_directory" has been deprecated in favor of
"file(MAKE_DIRECTORY ...)", so use that instead of the custom command. This
means the directories are created when the cmake config is generated, instead
of during the subsequent make steps.

Note: the mo-update target which builds the .mo files is only run when the
build is configured with ENABLE_NLS=ON. Both before and after this change,
none of the .mo files are rebuilt when ENABLE_NLS=OFF.

po/CMakeLists.txt

@@ -239,20 +239,14 @@ if(ENABLE_NLS)
 
 		foreach(DOMAIN ${DOMAINS})
 
-			add_custom_command(
-				OUTPUT ${CMAKE_SOURCE_DIR}/${LOCALEDIR}/${LINGUA}/LC_MESSAGES
-				COMMAND ${CMAKE_COMMAND} -E make_directory
-						${CMAKE_SOURCE_DIR}/${LOCALEDIR}/${LINGUA}/LC_MESSAGES
-				COMMENT "mo-update [${LINGUA}]: Creating locale directory."
-			)
+			file(MAKE_DIRECTORY ${CMAKE_SOURCE_DIR}/${LOCALEDIR}/${LINGUA}/LC_MESSAGES)
 
 			add_custom_command(
 				OUTPUT ${CMAKE_SOURCE_DIR}/${LOCALEDIR}/${LINGUA}/LC_MESSAGES/${DOMAIN}.mo
 				COMMAND ${GETTEXT_MSGFMT_EXECUTABLE}
 						${GETTEXT_MSGFMT_PARAMETER}
 						-o ${CMAKE_SOURCE_DIR}/${LOCALEDIR}/${LINGUA}/LC_MESSAGES/${DOMAIN}.mo ${LINGUA}.po
 				DEPENDS
-					${CMAKE_SOURCE_DIR}/${LOCALEDIR}/${LINGUA}/LC_MESSAGES
 					${PROJECT_SOURCE_DIR}/po/${DOMAIN}/${LINGUA}.po
 				WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}/po/${DOMAIN}
 				COMMENT "mo-update [${DOMAIN}-${LINGUA}]: Creating mo file."