Fix the linewrapping when updating a po file.

It seems when updating a po file it's not automatically line wrapped.
Use the same trick as autotools by writing to a temp file and move that
to the original.

changelog

@@ -45,6 +45,7 @@ Version 1.7.5+svn:
    * Avoid an assertion failure if haloes are disabled (bug #14297)
    * Fixed linked widgets not getting deregistered upon destruction
    * Fix huge fonts on storyscreens with tiny GUI
+   * Fix cmake po-update not doing line wrapping properly
 
 Version 1.7.5:
  * Campaigns:

po/CMakeLists.txt

@@ -276,9 +276,14 @@ if(ENABLE_POT_UPDATE_TARGET)
 				# still works.
 				OUTPUT ${PROJECT_SOURCE_DIR}/po/${DOMAIN}/${LANG}.po.lang.dummy
 				COMMAND ${GETTEXT_MSGMERGE_EXECUTABLE} 
-						--backup=none
-						-U ${LANG}.po 
+						${LANG}.po
 						${DOMAIN}.pot
+						-o ${LANG}.po.tmp
+				COMMAND ${CMAKE_COMMAND} -E copy
+						${LANG}.po.tmp
+						${LANG}.po
+				COMMAND ${CMAKE_COMMAND} -E remove
+						${LANG}.po.tmp
 				WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}/po/${DOMAIN}
 				COMMENT "po-update [${DOMAIN}-${LANG}]: Updated po file."
 			)