Work around compilation with gcc-4.8, render nans in c++14 mode only

Author: sizmailov

include/pybind11/cast.h

@@ -1871,7 +1871,7 @@ struct arg_v : arg {
     arg_v(const arg &base, T &&x, const char *descr = nullptr)
         : arg_v(arg(base), std::forward<T>(x), descr) { }
 
-#if __cplusplus >= 201402L
+#ifdef PYBIND11_CPP14
 #define ARGV_NAN_DEFAULT_OVERLOAD(FLOAT_TYPE) \
     arg_v(const arg &base, FLOAT_TYPE x) \
         : arg_v(arg(base), x, std::isnan(x) ? "numpy.nan" : nullptr) {}

tests/test_kwargs_and_defaults.cpp

@@ -35,7 +35,7 @@ TEST_SUBMODULE(kwargs_and_defaults, m) {
     m.def("kw_func_udl_z", kw_func, "x"_a, "y"_a=0);
 
     m.def("kw_func_float_123", kw_func_float, "x"_a=1, "y"_a=2, "z"_a=3);
-#if __cplusplus >= 201402L
+#ifdef PYBIND11_CPP14
     m.def("kw_func_float_nan", kw_func_float,
         "x"_a=std::numeric_limits<float>::quiet_NaN(),
         "y"_a=std::numeric_limits<double>::quiet_NaN(),