Update Mac support for 10.8 and 10.9 - need to review about earlier versions

cmr=v1.7.4:reviewer=ompi-gk1.7

This commit was SVN r30215.

README

@@ -128,7 +128,7 @@ General notes
   - Linux (various flavors/distros), 32 bit, with gcc
   - Linux (various flavors/distros), 64 bit (x86), with gcc, Absoft,
     Intel, and Portland (*)
-  - OS X (10.5, 10.6, 10.7), 32 and 64 bit (x86_64), with gcc and
+  - OS X (10.5, 10.6, 10.7, 10.8, 10.9), 32 and 64 bit (x86_64), with gcc and
     Absoft compilers (*)
 
   (*) Be sure to read the Compiler Notes, below.