v4.3

This version adds support for older versions of MacOS (see issue #1). Previously, the font metric code assumed that the CoreText framework was available on MacOS and did not test for its existence (nor allow the possibility of using other *nix options of Xft or Zlib on MacOS). Now, the presence of CoreText is explicitly tested, and Xft and Zlib are used as fallbacks if CoreText is not available (and Xft or Zlib is available).