AC_CONFIG_FILE([d1/foo:d2/foo]) triggers error messages when

AC_CONFIG_FILE([d1/foo:d2/foo]) triggers error messages when
computing the absolute path to d1 in the source hierarchy: it may
not exist at all.  So don't cd into it.
From Alexandre Duret-Lutz.
http://mail.gnu.org/archive/html/bug-autoconf/2003-10/msg00205.html
* lib/m4sugar/m4sh.m4 (AS_SET_CATFILE): New.
From Paul Eggert, but named after Perl's IO::Spec->catfile.
* doc/autoconf.texi (Programming in M4sh): Document.
* lib/autoconf/status.m4 (_AC_SRCPATHS): Use it.