gtkdoc-rebase detection troubles

[Stefan Kost <ensonic hora-obscura de>]

hi,

since the introduction of gtkdoc-rebase, I am patching the gtk-doc.make back and
forth to find a portable way to detect its existence. Right this is what we do:

-which gtkdoc-rebase >/dev/null && gtkdoc-rebase ...

There is a '-' to make the rule not fail. which returns false if it does not
find the command in the path (well that was the plan) and if found executes it.

On solaris and macos which does not return false if the command is not found:
http://bugzilla.gnome.org/show_bug.cgi?id=565322
http://bugzilla.gnome.org/show_bug.cgi?id=562310

For history read:
http://bugzilla.gnome.org/show_bug.cgi?id=508897
http://bugzilla.gnome.org/show_bug.cgi?id=559281
http://bugzilla.gnome.org/show_bug.cgi?id=501066
http://bugzilla.gnome.org/show_bug.cgi?id=501617

Any ideas how to write this so that everyone in this world is happy?

Stefan