Fix Duktape's C++11 detection for MSVC.

[jrlanglois]

Duktape does a faulty check for MSVC toolchains, specifically because MSVC has never supported the __cplusplus macro (which Duktape is using).

It would be better to define the problematic macro up front, outside of Duktape:

#define DUK_F_CPP11 (JUCE_CXX14_IS_AVAILABLE) //Note that parentheses might be needed

[nick-thompson]

:+1: cool with me! That's perhaps worth opening an issue for in the Duktape repo as well