Ensure that sys.prefix can reliably be found

on OSX. This fixes a small issue that was exposed
by running test_subprocess through regrtest (and
hence in a subdirectory).

Without this patch running python.exe from the
build tree will fail when these tree conditions
are true:
1) the CWD is not the root of build tree
2) python.exe is found through $PATH
3) the framework is not yet installed

Modules/getpath.c

@@ -552,7 +552,7 @@ calculate_path(void)
         if (!ismodule(argv0_path)) {
                 /* We are in the build directory so use the name of the
                    executable - we know that the absolute path is passed */
-                wcsncpy(argv0_path, prog, MAXPATHLEN);
+                wcsncpy(argv0_path, progpath, MAXPATHLEN);
         }
         else {
                 /* Use the location of the library as the progpath */