Configure: suppress $multilib with non-system $prefix [from HEAD].

my $arflags = $fields[$idx_arflags];

my $multilib = $fields[$idx_multilib];

# if $prefix/lib$multilib is not an existing directory, then

# assume that it's not searched by linker automatically, in

# which case adding $multilib suffix causes more grief than

# we're ready to tolerate, so don't...

$multilib="" if !-d "$prefix/lib$multilib";

$libdir="lib$multilib" if $libdir eq "";

$cflags = "$cflags$exp_cflags";