Use compiler warnings from gnome-common

Rather than hardcoding the compiler warnings in autogen.sh, use the ones
from gnome-common, which are passed to the C compiler during configure
to check that they are supported.
Fix missing prototypes caught by gcc
Use gnome-common deprecation macro
Fix gcc strict-prototypes warnings
Avoid gcc nested-externs warnings
Fix function pointer prototype in struct definition
Avoid gcc void pointer arithmetic warnings
Fix many gtk-doc warnings
Add deprecation guards for gtk-doc build