Fix cross compiling issue in setup.py, ensure that lib_dirs and inc_dirs are
defined in cross compiling mode, too.
diff --git a/setup.py b/setup.py
index 8a9ba5f..8125efc 100644
--- a/setup.py
+++ b/setup.py
@@ -506,6 +506,9 @@
                 '/lib', '/usr/lib',
                 ]
             inc_dirs = self.compiler.include_dirs + ['/usr/include']
+        else:
+            lib_dirs = []
+            inc_dirs = []
         exts = []
         missing = []