Add --diagnostics option

It will cause compton to print out some rudimentary diagnostics.

Also small improvements of the meson.build.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/diagnostic.h

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) 2018 Yuxuan Shui <yshuiv7@gmail.com>
+
+#pragma once
+
+typedef struct session session_t;
+
+void print_diagnostics(session_t *);