autoconf: exec_prefix should default to $prefix not $ac_default_prefix

autoconf: exec_prefix should default to $prefix not $ac_default_prefix

Otherwise "./configure --prefix=/usr" still ends up with exec_prefix=/usr/local
and therefore LIBDIR, PRIVATE_PREFIX and PRIVATE_BINDIR still point to
/usr/local/foo.

This was broken in 54f28913a2b3 "autoconf: xen: move standard path variables to
config/Paths.mk.in" which tried to add code to expand exec_prefix earlier.

Reported-by: Julien Grall <julien.grall@linaro.org>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Luis R. Rodriguez <mcgrof@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>