Fix installation when --root is specified

setup.py

@@ -7,6 +7,7 @@ import os
 import sys
 import platform
 import imp
+import argparse
 
 version = imp.load_source('version', 'lib/version.py')
 
@@ -16,8 +17,12 @@ if sys.version_info[:3] < (2, 7, 0):
 data_files = []
 
 if platform.system() in ['Linux', 'FreeBSD', 'DragonFly']:
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--root=', dest='root_path', metavar='dir', default='/')
+    opts, _ = parser.parse_known_args(sys.argv[1:])
     usr_share = os.path.join(sys.prefix, "share")
-    if not os.access(usr_share, os.W_OK):
+    if not os.access(opts.root_path + usr_share, os.W_OK) and \
+       not os.access(opts.root_path, os.W_OK):
         if 'XDG_DATA_HOME' in os.environ.keys():
             usr_share = os.environ['$XDG_DATA_HOME']
         else: