Use stat -f for MacOSX (trilinos/Trilinos#7881)

I have not been able to test that installation with this group and/or perms
modification code works on MacOSX but it was reported in
trilinos/Trilinos#7881 that calling 'stat -f %U <file>' works correctly.  (I
hope to be able to test this later as part of general MacOSX testing for
TriBITS.)

tribits/core/installation/set_installed_group_and_permissions.cmake.in

@@ -28,7 +28,13 @@ ENDFUNCTION()
 
 FUNCTION(SET_DIR_OWNER_AND_PERMS  dirPath  recurseFlag)
 
-  EXECUTE_PROCESS(COMMAND stat -c %U "${dirPath}"
+  IF (CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+    SET(STAT_ARG "-f")  # MacOSX stat
+  ELSE()
+    SET(STAT_ARG "-c")  # BinUtils stat
+  ENDIF()
+
+  EXECUTE_PROCESS(COMMAND stat ${STAT_ARG} %U "${dirPath}"
     OUTPUT_STRIP_TRAILING_WHITESPACE
     OUTPUT_VARIABLE  dirOwner)