gamemode: use zig build

libs/gamemode/build.zig

@@ -0,0 +1,14 @@
+const std = @import("std");
+
+pub const pkg = std.build.Pkg{
+    .name = "gamemode",
+    .source = .{ .path = thisDir() ++ "/gamemode.zig" },
+};
+
+pub fn link(step: *std.build.LibExeObjStep) void {
+    step.addIncludeDir(comptime thisDir() ++ "/upstream/include");
+}
+
+fn thisDir() []const u8 {
+    return std.fs.path.dirname(@src().file) orelse ".";
+}