Use system cppgir

https://github.com/desktop-app/cmake_helpers/issues/282
https://github.com/desktop-app/cmake_helpers/pull/305
--- tdesktop-5.14.3-full.orig/cmake/external/glib/CMakeLists.txt
+++ tdesktop-5.14.3-full/cmake/external/glib/CMakeLists.txt
@@ -7,13 +7,6 @@
 add_library(external_glib INTERFACE IMPORTED GLOBAL)
 add_library(desktop-app::external_glib ALIAS external_glib)
 
-block()
-    set(BUILD_TESTING OFF)
-    set(BUILD_DOC OFF)
-    set(BUILD_EXAMPLES OFF)
-    add_subdirectory(cppgir EXCLUDE_FROM_ALL)
-endblock()
-
 include(generate_cppgir.cmake)
 generate_cppgir(external_glib Gio-2.0)
 
--- tdesktop-5.14.3-full.orig/cmake/external/glib/generate_cppgir.cmake
+++ tdesktop-5.14.3-full/cmake/external/glib/generate_cppgir.cmake
@@ -4,6 +4,8 @@
 # For license and copyright information please follow this link:
 # https://github.com/desktop-app/legal/blob/master/LEGAL
 
+find_package(CppGir REQUIRED)
+
 function(generate_cppgir target_name gir)
     set(cppgir_loc ${cmake_helpers_loc}/external/glib/cppgir)