Different base URL for @xref?

[Alfred M. Szmidt]

Is it possible to supply a different base URL when using @xref?

@xref{argp, , Parsing Program Options with Argp, libc, The GNU C
Library Reference Manual}, for details.

will produce a local reference to the `libc.html#argp', which isn't
entierly useful if you are refering to external sources or sources
living in a different directory; this only happens for HTML output.

Currently, I'm doing ugly things like:

@ifhtml
See @url{http://www.gnu.org/s/libc/manual/html_node/Argp.html,Parsing Program 
Options with Argp}, for details. 
@end ifhtml
@ifnothtml
@xref{argp, , Parsing Program Options with Argp, libc, The GNU C Library 
Reference Manual}, for details.
@end ifnothtml

And would like to make it a bit nicer.