Python: limited support for osx w/ homebrew

Looks for xrootd include and library directories where they are found on
OS X/Darwin systems where xrootd has been installed via homebrew.

Only works if the bindings version match the xrootd version installed
via homebrew.

Comments on the installation procedure for OSX in README.rst

bindings/python/README.rst

@@ -0,0 +1,16 @@
+Prerequisites (incomplete): xrootd
+
+# Installing on OSX
+Setup should succeed if:
+ - xrootd is installed on your system
+ - xrootd was installed via homebrew
+ - you're installing the bindings package with the same version number as the
+   xrootd installation (`xrootd -v`).
+
+If you have xrootd installed and the installation still fails, do
+`XRD_LIBDIR=XYZ; XRD_INCDIR=ZYX; pip install xrootd`
+where XYZ and ZYX are the paths to the XRootD library and include directories on your system.
+
+## How to find the lib and inc directories
+To find the library directory, search your system for "libXrd*" files.
+The include directory should contain a file named "XrdVersion.hh", so search for that.

bindings/python/setup_pypi.py

@@ -1,6 +1,7 @@
 from distutils.core import setup, Extension
 from distutils import sysconfig
 from os import getenv, walk, path, path, getcwd, chdir
+from platform import system
 import subprocess
 
 # Remove the "-Wstrict-prototypes" compiler option, which isn't valid for C++.
@@ -19,13 +20,23 @@
     elif filename.endswith('.hh'):
       depends.append(path.join(dirname, filename))
 
-xrdlibdir = getenv( 'XRD_LIBDIR' ) or '/usr/lib'
-xrdincdir = getenv( 'XRD_INCDIR' ) or '/usr/include/xrootd'
-
 # Get package version
 with open ('VERSION_INFO') as verfile:
     version = verfile.read().strip()
 
+def getincdir_osx():
+    # Assume xrootd was installed via homebrew
+    return '/usr/local/Cellar/xrootd/{0}/include/xrootd'.format(version)
+
+def getlibdir():
+    return (system() == 'Darwin' and '/usr/local/lib') or '/usr/lib'
+
+def getincdir():
+    return (system() == 'Darwin' and getincdir_osx()) or '/usr/include/xrootd'
+
+xrdlibdir = getenv( 'XRD_LIBDIR' ) or getlibdir()
+xrdincdir = getenv( 'XRD_INCDIR' ) or getincdir()
+
 print 'XRootD library dir: ', xrdlibdir
 print 'XRootD include dir: ', xrdincdir
 print 'Version:            ', version