FindXrootd.cmake fix, 1.0 branch (2017.03.29.)

Made the FindXrootd.cmake module a little smarter. It now looks for the libraries not only under lib64/, but also under lib/. This should help in future ports to other platforms.

The main addition is that the module now sets the XROOTD_BINARY_PATH variable, which is later used by the environment setup script generation to set up the $PATH environment variable. Fixing the runtime environment, so that it would pick up the xrd executables from the right place.

This is to fix the issue described in ATLINFR-1631.