build: Remove dependency on gnome-common

Inline the relevant code from gnome-autogen.sh (there isn’t much of it),
and do the same with the GNOME macros in configure.ac.

Copy the gnome-compiler-flags.m4 macro in-tree. It’s licenced under
GPLv2+, which is fine.
clang-plugin: Clarify a warning about (allow-none) annotations
bugs: File some new feature idea bugs
clang-plugin: Fix #define used to enable debug output
clang-plugin: Improve WARN_EXPR print format
clang-plugin: Add missing #includes to gir-manager.h