Import vsnprintf from libiberty if not available.

Import vsnprintf from libiberty if not available.

vasprintf and vsnprintf are not available on LynxOS, at least not on
version 4.0.0.  This patch updates the configury to import them from
libiberty if not available out of the box.

gdbserver/
        * Makefile.in (vasprintf.o, vsnprintf.o): New rules.
        * configure.ac: Add check for vasprintf and vsnprintf.
        * configure, config.in: Regenerate.
        * server.h (vasprintf, vsnprintf): Add conditional declarations.