Update build instructions for macOS

[filipnavara]

The current build instructions for builds with X11 support cause an outdated cairo to be used. There's Cairo 1.14.6 distributed with XQuartz and the suggested build command (in README.md) picks it up instead of the cairo build from Homebrew.

To pick-up the Homebrew Cairo it needs to be changed from

PKG_CONFIG_PATH=/opt/X11/lib/pkgconfig ./autogen.sh --prefix=YOUR_PREFIX

to

PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/opt/X11/lib/pkgconfig  ./autogen.sh --prefix=YOUR_PREFIX

Unfortunately the default cairo Homebrew build is built without X11 support, so furthermore the following is needed:

brew uninstall cairo
brew install --with-x11 cairo

[filipnavara]

Homebrew killed the --with-x11 option on the cairo package. Unfortunately there's no other easy option to build this configuration.

[NattyNarwhal]

Do people still use MacPorts? I also know one of the pkgsrc maintainers for macOS, if that helps.

[filipnavara]

I think at this point the only sane option is to mark it as unsupported configuration and remove it from the instructions.