MacOSX doesn't define HOST_NAME_MAX in unistd.h, which made freeglut fail to

build. Now, if the definition is missing, we'll just define it arbitrarilly as
255, which should be a safe value. This is just needed for an obscure X11 WM
protocol.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1746 7f0cb862-5218-0410-a997-914c9d46530a