build: autoconf: fix libdir detection

build: autoconf: fix libdir detection

If user specifies a libdir it is used, if no libdir is specified
configure checks if $exec_prefix/lib64 is a directory and uses that,
if not lib is used.

$prefix is set by passing the command line option or by default when
calling AC_OUTPUT, but since AC_OUTPUT is called at the end, this is
not really helpful, so we have to set $exec_prefix manually to the
correct value, either $prefix if different than NONE or
$ac_default_prefix.

Signed-off-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>