Impl. a way to ignore software in the system, e.g. set ATLAS=None to pretend that Atlas libraries are not availiable.

diff --git a/scipy_distutils/system_info.py b/scipy_distutils/system_info.py
index a6bf3db..ea27f73 100644
--- a/scipy_distutils/system_info.py
+++ b/scipy_distutils/system_info.py

@@ -4,6 +4,7 @@
 information about various resources (libraries, library directories,
 include directories, etc.) in the system. Currently, the following
 classes are available:
+
   atlas_info
   atlas_threads_info
   atlas_blas_info
@@ -21,8 +22,8 @@
   blas_src_info
   numpy_info
   numarray_info
-  boost_python
-  agg2
+  boost_python_info
+  agg2_info
   wx_info
   gdk_pixbuf_xlib_2_info
   gdk_pixbuf_2_info
@@ -42,6 +43,11 @@
   distutils.setup keyword arguments. If info_dict == {}, then the
   asked resource is not available (system_info could not find it).
 
+  Several *_info classes specify an environment variable to specify
+  the locations of software. When setting the corresponding environment
+  variable to 'None' then the software will be ignored, even when it
+  is available in system.
+
 Global parameters:
   system_info.search_static_first - search static libraries (.a)
              in precedence to shared ones (.so, .sl) if enabled.
@@ -333,6 +339,9 @@
         dirs = self.cp.get(section, key).split(os.pathsep)
         if self.dir_env_var and os.environ.has_key(self.dir_env_var):
             d = os.environ[self.dir_env_var]
+            if d=='None':
+                print 'Disabled',self.__class__.__name__,'(%s is None)' % (self.dir_env_var)
+                return []
             if os.path.isfile(d):
                 dirs = [os.path.dirname(d)] + dirs
                 l = getattr(self,'_lib_names',[])