Align FindGettext with CMP002 policy

Otherwise some apps fail to build with kdelibs >= 4.14.11:

CMake Error at /usr/share/kde4/apps/cmake/modules/FindGettext.cmake:232 (ADD_CUSTOM_TARGET):
  add_custom_target cannot create target "pofiles" because another target
  with the same name already exists.  The existing target is a custom target
  created in source directory
  "/home/hrvoje/rpmbuild/BUILD/skanlite-1.1/po/be".  See documentation for
  policy CMP0002 for more details.
Call Stack (most recent call first):
  po/zh_CN/CMakeLists.txt:2 (GETTEXT_PROCESS_PO_FILES)

REVIEW: 125561