configure: make --prefix=/usr and '' identical

[DJ Delorie]

Joseph Myers <josmyers@redhat.com> writes:
> Does the AC_PREFIX_DEFAULT interface require such an autoconf-time 
> constant at an early point in configure.ac?

The problem is that we pull in sysdeps config files from the most
specific to the generic, so setting --prefix in the linux config means
it won't be set in the linux/x86_64 config, which is where we set
libc_cv_complocaledir

If this is not acceptable, reverting my original patch makes more
sense.  It's just annoying to have to type --prefix=/usr ALL THE TIME.

> (The proper default for Hurd is empty - no /usr - as used in
> build-many-glibcs.py.)

The proper incantation for Hurd is --prefix= which still does the right
thing.  My change only affects if you omit --prefix= completely, which
previously did the wrong thing for Hurd too (defaulting to /usr/local)