Update setup.py

Fixes issues ThomasLecocq#6 and ThomasLecocq#8. Followed the solution in the obspy issue mentioned in ThomasLecocq#6.

setup.py

@@ -67,7 +67,7 @@ def export_symbols(*path):
 EXTERNAL_LIBS = False
 # adds --with-system-libs command-line option if possible
 def add_features():
-    if 'setuptools' not in sys.modules:
+    if 'setuptools' not in sys.modules or not hasattr(setuptools, 'Feature'):
         return {}
 
     class ExternalLibFeature(setuptools.Feature):