updated get_prefix

with system python branch from @2ashish24 things get installed somewhere else now on Mac
this catches it and fix error on json from vcs

Author: doutriaux1

CMake/cdat_modules_extra/install.py.in

@@ -262,9 +262,15 @@ def get_version():
   return Version
 
 def get_prefix():
-  import os
+  import os,sys
   try:
-    return os.environ["UVCDAT_SETUP_PATH"]
+    uv_setup_pth = os.environ["UVCDAT_SETUP_PATH"]
+    if os.uname()[0] == "Darwin":
+      uv_setup_pth = os.path.join(uv_setup_pth,
+          "Library","Frameworks","Python.framework","Versions",
+          "%i.%i" % (sys.version_info.major,sys.version_info.minor)
+          )
+    return uv_setup_pth
   except KeyError:
     raise RuntimeError("UVCDAT environment not configured. Please source the setup_runtime script.")