cmake: prefer system gamemode library

CMakeModules/Findgamemode.cmake

@@ -0,0 +1,15 @@
+# SPDX-FileCopyrightText: 2023 yuzu Emulator Project
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+find_package(PkgConfig QUIET)
+pkg_search_module(GAMEMODE QUIET IMPORTED_TARGET gamemode)
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(gamemode
+    REQUIRED_VARS GAMEMODE_INCLUDEDIR
+    VERSION_VAR GAMEMODE_VERSION
+)
+
+if (gamemode_FOUND AND NOT TARGET gamemode::headers)
+    add_library(gamemode::headers ALIAS PkgConfig::GAMEMODE)
+endif()