configdata.pm: ensure $prefix is defined
Fixes
@@ -2444,6 +2444,7 @@ _____
my $prefix = '';
$prefix = $config{cross_compile_prefix}
if grep { $var eq $_ } @user_crossable;
+ $prefix //= '';
print ' ',$var,' ' x (16 - length $var),'= ',
(ref $config{$makevars{$var}} eq 'ARRAY'
? join(' ', @{$config{$makevars{$var}}})
