Installation path fix for UNIX systems

[giuliods]

While compiling and installing Ironwail on Linux the Makefile wasn't able to create the directory and put the files in the folders correctly.

This pull request should do these things:

• it assures to create the folder quake in the correct path. The
  folder /usr/local/games is common but not always present on every distribution and it is
  necessary to create it if not present.
• I created a folder /usr/local/share for the pak so the files useful for
  the binary are available on the standard path rather than on games.
• And I created a uninstall phony to remove everything.

This required some modification to the pak position in quakedef.h which are defined only
for __unix