build-sys: rewrite autogen.sh

Start it from scratch:
- no need to define ACLOCAL_FLAGS again with same value
- USE_GNOME2_MACROS is deprecated for 10y
- cd "$srcdir" is wrong: you can't autogen.sh out-of-tree because of this
- srcdir=. is already a fallback in gnome-autogen.sh
- there is no need to override $srcdir, so define it without condition
- there is no gtk-doc, remove --enable-gtk-doc
- remove copyright
build-sys: remove unused gtk-doc