[libstdc++] Use LD_RUN_PATH when linking abi_check

[Rainer Orth]

Phil Edwards <phil@jaj.com> writes:

> Rather than trying to find an -R/-rpath/-Wl,--rpath solution that would
> work on all systems, let's just trust the linker to Do The Right Thing when
> LD_RUN_PATH is present.  I've verified that this works for me, additional
> reports would be welcome.

This doesn't work on Tru64 UNIX and IRIX 6 which both lack support for
LD_RUN_PATH.  I think that instead we should use LD_LIBRARY_PATH (and it's
LD_LIBRARYN32_PATH/LD_LIBRARY64_PATH companions on IRIX 6) as we do all the
time in the testsuite.  This is a tested solution and seems to work
everywhere.

	Rainer