This builds urxvt-unicode with the patch from https://github.com/blueyed/rxvt-unicode/tree/display-wide-glyphs to display glyhps that are too wide instead of a square.
urxvt considers the font to be broken, e.g. if the glyph is wider than the number of cells that wcwidth(3) reports. This gets better with Unicode 9 (since e.g. more characters are assigned a width of 2), but it does not help with characters from the Private Use Area (e.g. Font Awesome).
More information : https://github.com/blueyed/PKGBUILD-rxvt-unicode-wide
Place directory in your local overlay and emerge rxvt-unicode
If you don't already have ~amd64 for rxvt-unicode then you should put this in package.accept_keywords :
x11-terms/rxvt-unicode ~amd64
In your package.use you have to enable wide-glyphs USE flag