tests: avoid a spurious failure on MacOS X 10.6.8

tests: avoid a spurious failure on MacOS X 10.6.8

Fixes automake bug#14706.

* t/depcomp2.sh: Strip, from the redirected ./configure stderr, the
possible error message "rm: conftest.dSYM: is a directory", generated
by cleanup code that doesn't cater to the existence of *.dSYM
directories sometimes created by the compiler on MacOS X.  This
"massaging" of ./configure stderr is legitimate, since the spurious
error message is due not to automake-related code, but to a know
buglet/limitation of either Autoconf or Mac OS X bundles gcc:
<http://lists.gnu.org/archive/html/bug-autoconf/2007-11/msg00017.html>
Actually, from that link it appears that the original Autoconf issue
had been fixed, but it must have been re-introduced in the meantime :-(

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>