Modified setup.py's EXTRA_OBJECTS setting to use an empty set ([] vs '')

when the variable isn't found in the check.  The empty value is
problematic for distutils, but NCAR's compiler wrappers eliminate the
empty space, whereas directly calling the compilers does not - this
leads to build problems on other systems.  (Usually hidden because it's
installed via Conda in most cases.)

I'm very bad at python, so my terminology above is likely incorrect, but
I'm quite sure the diagnosis is right.  :-)

setup.py

@@ -357,7 +357,7 @@ def set_curl_libs():
 try:
   EXTRA_OBJECTS = [os.environ["EXTRA_OBJECTS"]]
 except:
-  EXTRA_OBJECTS = ''
+  EXTRA_OBJECTS = []
 
 #
 # Done with environment variables.