....
if($Config{nvsize} == 8)   { $nvtype = 'LU_NV_IS_64_BIT'  } # double or 8-byte long double
elsif(length(sqrt 2) > 25) { $nvtype = 'LU_NV_IS_128_BIT' } # IEEE long double or __float128
else                       { $nvtype = 'LU_NV_IS_80_BIT'  } # extended precision long double
$defines .= " -D$nvtype";
....
WriteMakefile (
 ....
 DEFINE => $defines,
 ....
);