gtkdocize: Fix GTK_DOC_CHECK argument handling

After commit 1401aebc625dd1c7c39457d3455dce3019ded0be, any extra
arguments passed to gtkdocize by GTK_DOC_CHECK in configure.ac were
ignored. This broke because the arguments to GTK_DOC_CHECK are injected
into the argument for gtkdocize, but the rearrangement to add --srcdir
support pushed the injection down to after where the arguments were
parsed.

Fix the argument handling by checking for a --srcdir agument (and for
GTK_DOC_CHECK) before parsing other arguments.

https://bugzilla.gnome.org/show_bug.cgi?id=707426