Avoid building the the python wrapper shared lib again if it already exists...

Avoid building the the python wrapper shared lib again if it already exists (it must be asked explicitely)

It completes a414c959.

wrapper/python/setup.py.in

@@ -2,6 +2,7 @@
 # @PYFAUST_LICENSE_HEADER@
 
 from setuptools import setup, Extension
+import sysconfig
 #from distutils.core import setup, Extension
 from Cython.Build import cythonize
 import numpy
@@ -40,6 +41,11 @@ elif platform == 'darwin':
 elif platform == 'win32':
     lib_ext = 'dll'
 
+lib_path = "_FaustCorePy"+sysconfig.get_config_vars()['EXT_SUFFIX']
+from os.path import exists, dirname, join
+if sys.argv[1] == "build_ext" and exists(join(dirname(sys.argv[0]),lib_path)):
+    print("The shared library", lib_path, "already exists, please delete it to build again.")
+    exit(0)
 
 PyFaust = Extension('_FaustCorePy',
                     sources = @PY_EXT_SOURCES@,
@@ -79,3 +85,5 @@ setup(
         'pyfaust': [ 'data/*.mat', 'license*.txt', 'lib/*.'+lib_ext]
     }
 )
+
+