INSTALL requirements could be better

[rogpeppe]

The INSTALL file says "if you can compile any X apps at all, you'll be able to compile xclip". That doesn't appear to be quite true (at least on my stock Ubuntu 16.04 platform).

I got it compiling by running the following (after judicious running of apt-file to find out what might work):

sudo apt install libx11-dev
sudo apt install libxmu-headers
sudo apt install libxt-dev
sudo apt install libxmu-dev

I'm not sure if all of these are necessary, or whether one of the later ones might be sufficient to include one of the earlier ones.