build-sys: properly report missing gtk-doc

This brings the check for ENABLE_GTK_DOC in line with
HAVE_INTROSPECTION and other similar checks. Only
the status line that is printed with uninstalled
gtk-doc is changed.

https://bugs.freedesktop.org/show_bug.cgi?id=63108
build-sys: fix gtkdocize check

gtkdocize: GTK_DOC_CHECK not called in configure.ac

Fixup for 6581f00f7ea.