chicken @4.13.0 conflicts with mono @6.12.0.122

[cooljeanius (Eric Gallager)]

I believe there have been reports of this conflict causing build failures for other ports, but none for the actual conflict itself. In any case, they both try to install a csc executable:

x ./opt/local/bin/chicken-bug
DEBUG: Image error: /opt/local/bin/csc is being used by the active mono port.  Please deactivate this port first, or use 'port -f activate chicken' to force the activation.
    invoked from within
"throw registry::image-error "Image error: $file is being used by the active [$owner name] port.  Please deactivate this port first, or use 'port -f ac..."
    ("foreach" body line 45)
    invoked from within
"foreach file $imagefiles {
                set srcfile "${extracted_dir}${file}"

                # To be able to install links, we test if we can lst..."
    invoked from within
"registry::write {
            foreach file $imagefiles {
                set srcfile "${extracted_dir}${file}"

                # To be able to instal..."
    invoked from within
"try {
        registry::write {
            foreach file $imagefiles {
                set srcfile "${extracted_dir}${file}"

                # To be ..."
    (procedure "_activate_contents" line 21)
    invoked from within
"_activate_contents $requested $rename_list"
    (procedure "activate" line 63)
    invoked from within
"activate $name "" "" 0 $optionslist"
    (procedure "portimage::activate_composite" line 3)
    invoked from within
"portimage::activate_composite $portname $composite_version [array get options]"
Error: port activate failed: Image error: /opt/local/bin/csc is being used by the active mono port.  Please deactivate this port first, or use 'port -f activate chicken' to force the activation.

This conflict has been around so long that I have little hope of it being properly fixed, but at the very least the conflict could be marked.