textproc/gtk-doc: make configure dependency on highlight explicit

If textproc/source-highlight is installed when you run 'make -C
textproc/gtk-doc configure', the configure phase will detect source-highlight
first before highlight. This makes gtk-doc implicitly dependent on
source-highlight. If source-highlight is later removed, gtk-doc is broken
(config.py contains a now invalid reference to source-highlight).

This is solved by specifying --with-highlight=highlight for configure.

PR: 240536
Submitted by: John Hein <jcfyecrayz@liamekaens.com>