gflags: CMake 4+ compatibility (#851)

Set `CMAKE_POLICY_VERSION_MINIMUM=3.5` to make the project work with
CMake 4+.

This fixes the following error:
```
CMake Error at CMakeLists.txt (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.
```

Author: NeroBurner

cmake/projects/gflags/hunter.cmake

@@ -53,10 +53,18 @@ hunter_add_version(
     4d42470afb7236fb0cf90f8bbb0cec588073c17c
 )
 
+if(HUNTER_glfags_VERSION VERSION_LESS_EQUAL 2.2.2)
+    # CMake 4.0+ compatibility with older glfags packages
+    set(_hunter_glfags_cmake_compatibility_flag "CMAKE_POLICY_VERSION_MINIMUM=3.5")
+else()
+    set(_hunter_glfags_cmake_compatibility_flag "")
+endif()
+
 hunter_cmake_args(
     gflags
     CMAKE_ARGS
     REGISTER_INSTALL_PREFIX=OFF
+    ${_hunter_glfags_cmake_compatibility_flag}
 )
 
 hunter_pick_scheme(DEFAULT url_sha1_cmake)