Cmake build: Omit "PRE_BUILD" in add_custom_command()

Cmake build: Omit "PRE_BUILD" in add_custom_command()

According to cmake warning output while configuring:
CMake Error (dev) at po/CMakeLists.txt:72 (ADD_CUSTOM_COMMAND):
  The following keywords are not supported when using
  add_custom_command(OUTPUT): PRE_BUILD.

  Policy CMP0175 is not set: add_custom_command() rejects invalid arguments.
  Run "cmake --help-policy CMP0175" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.
Call Stack (most recent call first):
  po/CMakeLists.txt:106 (add_gettext_python)
This error is for project developers. Use -Wno-error=dev to suppress it.