autoreconf: don’t fail if gtkdocize is unavailable (#110503)

autoreconf: don’t fail if gtkdocize is unavailable (#110503)

Since 2.70, autoreconf has run gtkdocize when it detects a use of the
macro GTK_DOC_CHECK in the configure.ac.  Some projects (e.g. per the
bug report, gnutls and harfbuzz) were using a clever hack with
m4_ifdef to allow people to build the project without having gtkdocize
installed, if they didn’t care about building the documentation; the
change to autoreconf broke this hack.

Attempt to make the hack work again, by only running gtkdocize if that
command is actually installed; autoreconf will not error out if the
macro is detected but the command is unavailable, similar to the
existing treatment of autopoint.

Problem reported by Marvin Scholz.

* bin/autoreconf.in (autoreconf_current_directory): Do not error out
  if gtkdocize is wanted but unavailable.