gtkdoc-rebase error in ORBit2 on Darwin

["Joachim Kuebart" <joachim.kuebart%gmx.net@localhost>]

Hi,

when building ORBit2 on Darwin/PPC, I'm having the same problem as reported in 
pkg/40031: the ORBit2 Makefile also uses "which" to check for the presence of 
gtkdoc-rebase, which fails on Darwin if gtk-doc isn't installed. As a remedy, 
TOOLS_NOOP=gtkdoc-rebase was added for SunOS and Darwin in rev. 1.152 of 
devel/glib2/Makefile.

I think the same needs to be done for the ORBit2 package Makefile.

Best regards,

Joachim Kuebart