Add TODO list

Fix compiler flags handling

Filter out -fPIE for libraries, make sure we always add include paths to
CFLAGS (use overrides), and append -fPIC only for libraries.

Also override GObject introspection called functions

Some functions that we need to override may also be called via GObject
introspection (e.g. when calling them from Python and other scripting
languages), so we need to make sure that also this method of calling
these functions causes our overrides to be called.

To this end, override the g_function_info_prep_invoker method to
replace the function pointers if necessary.