Fix clean target. (#32)

`PDALVERSION` is not set when executing `setup.py clean`.

Ref: #24

Author: sebastic

setup.py

@@ -156,7 +156,7 @@ def get_pdal_config(option):
     if os.name != 'nt':
         extra_compile_args += ['-g','-O0']
 
-if PDALVERSION < Version('2.0.0'):
+if PDALVERSION is not None and PDALVERSION < Version('2.0.0'):
     raise Exception("PDAL version '%s' is not compatible with PDAL Python library version '%s'"%(PDALVERSION, module_version))