automake: update gtk-doc.m4 to serial 2

Update gtk-doc.m4 infra to serial (version) 2.
Some packages start to need/ship with the new version and can't be
properly autoreconf'ed, like kmod 18+.
The file was picked up from kmod-18 itself actually.

[Peter: don't pull in host-pkgconf as it was already needed by v1]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>