Any documentation for "--tag" ?

[Phil Endecott]

Dear libtool experts,

I'm the author of a C++ Apache module which uses an Apache-supplied 
libtool in its build process.

Some time ago people started reporting the "unable to infer tagged 
configuration" error from libtool.  It seemed to depend on particular 
distribution / version issues, but I never saw it myself.  Someone 
worked out that the solution was to add "--tag=CXX", but I don't know 
how they worked that out since I have never found any documentation for 
this mysterious --tag option.

Having recently upgraded something I have now started to see this 
message myself, so I think it's time to work out what is going on.

So, my main question is, is there any documentation for "--tag"?  It 
doesn't seem to be mentioned in the online manual at http://www.gnu.org/software/libtool/manual.html.

Now as I understand it the libtool that is supplied by Apache is 
somehow customised by them.  So is there something that they should do 
so that their libtool automatically recognises C++ compilation without 
the need for --tag?

Many thanks,


Phil.