REGRESSION(280217@main) CMake build failure after bump to cmake 3.20

https://bugs.webkit.org/show_bug.cgi?id=275735

Reviewed by Justin Michaud.

Setting required CMake to a mininum >=3.19 enforces the policy CMP0112,
which states:

> Target file component generator expressions do not add target dependencies.

This leads to some dependencies missing for gir targets.

This commit makes GI targets explicitly depend on these previously
implicit deps.

Thanks to Daniel Kolesa for the suggested fix.

Also remove deprecated policy.

* CMakeLists.txt:

Canonical link: https://commits.webkit.org/280248@main

CMakeLists.txt

@@ -9,11 +9,6 @@
 cmake_minimum_required(VERSION 3.20)
 project(WebKit)
 
-# Remove this cmake_policy() after upgrading cmake_minimum_required() to 3.20.
-if (${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.20")
-    cmake_policy(SET CMP0116 NEW)
-endif ()
-
 # -----------------------------------------------------------------------------
 # Common configuration
 #------------------------------------------------------------------------------

Source/cmake/FindGI.cmake

@@ -243,7 +243,7 @@ function(GI_INTROSPECT namespace nsversion header)
         message(FATAL_ERROR "Target '${opt_TARGET}' was not defined")
     endif ()
 
-    set(gir_deps)
+    set(gir_deps ${opt_TARGET})
     set(gir_name "${namespace}-${nsversion}")
     set(gir_path "${CMAKE_BINARY_DIR}/${gir_name}.gir")
     set(typ_path "${CMAKE_BINARY_DIR}/${gir_name}.typelib")