Graph output does not handle Unicode on Windows

[cjee21]

Graph output currently does not render non-ASCII text on Windows unlike on Linux. On Windows, the default font selection of Graphviz/Pango/Cairo does not fallback to other fonts when the default font does not have the required character and just renders a 'tofu' instead.

One solution is to add the following before call to gvContext();

#ifdef _WIN32
    // Make PangoCairo use FontConfig so that non-ASCII text can fallback to other fonts and render properly on Windows
    SetEnvironmentVariableW(L"PANGOCAIRO_BACKEND", L"fc");
#endif // _WIN32

However, this causes one line to be printed to stderr:

Fontconfig error: Cannot load default config file: No such file: (null)