fix numeric_limits<long double>

If SPROUT_NO_CXX11_NUMERIC_LIMITS and __LDBL_DENORM_MIN__ is defined,
the definition of numeric_limits uses __builtin_huge_val, __builtin_nan
and __builtin_nans but I think that it should use __builtin_huge_vall,
__builtin_nanl and __builtin_nansl, respectively.
Merge pull request #67 from kariya-mitsuru/fix_limits

fix numeric_limits<long double>