OSX manpath

[veprbl]

xrootd OSX installation uses /man for man pages while this doesn't seem to be the natural way for currents OSX (on my Yosemite there is a /usr/share/man and /usr/local/share/man, but no /usr/man or /usr/local/man)

Major OSX port collections are currently working around this:

• macports: https://trac.macports.org/browser/trunk/dports/science/xrootd/files/patch-cmake-XRootDOSDefs.cmake.diff
• homebrew: https://github.com/Homebrew/homebrew/blob/5b0604c6c2910681fdf4b2821b8086a3a7878143/Library/Formula/xrootd.rb#L23

The solution seems to be to either apply macports patch or use GNUInstallDirs.