configure.ac, vars.am: Use host-specific link options for test programs.

It turns out that `libtool' spams an annoying warning message to the
terminal every time you call it with `-no-install' on a Windowsish or
Mac OSish system.  Since this is just intended to be an optimization and
developer-convenience feature, wind it down to `-no-fast-install' on the
affected platforms so as not to provoke these really annoying messages.