libass fix for PowerPC

[barracuda156]

1. Portfile must invoke macports-gcc-7, as gcc-4.2 fails to build it.

Adding this works:

compiler.whitelist macports-gcc-7

Otherwise (and perhaps better) we can just ban gcc-4.0 and gcc-4.2.

2. There is one problem with the source in ass_coretext.c:

static bool is_postscript_font_format(CFNumberRef cfformat)
{
    bool ret = false;
    int format;
    if (CFNumberGetValue(cfformat, kCFNumberIntType, &format)) {
        ret = format == kCTFontFormatOpenTypePostScript ||
              format == kCTFontFormatPostScript;
    }
    return ret;
}

Once kCTFontFormatOpenTypePostScript is removed, libass builds. I am not sure it is an optimal fix though. Opened a ticket here: ​https://github.com/libass/libass/issues/595