[PATCH] solve my libtool 1.5 tag woes in CVS libtool

[Paolo Bonzini]

This patch (relative to CVS libtool) provides a macro to define a tag 
based on an existing one.  This is necessary for compatibility, because 
it won't break if addition _LT_AC_TAGVAR's are defined. 

Compared to my previous solution of redefining AC_LIBTOOL_CONFIG 
temporarily and invoking the standard AC_LIBTOOL_LANG_*_CONFIG this has 
several advantages:
- it does not execute the tests twice
- it saves more than 100kb in the configure script
- you can define tags based on previously defined custom tags
- actually, the other approach also had to copy some _LT_AC_TAGVAR's 
manually (notably, compiler and CC)

Paolo