Fix CMakeLists.txt: do not override CMAKE_OSX_ARCHITECTURES

* CMakeLists.txt: Since latest OSX compiler (clang) provided by Xcode is
not accept "-arch ppc"; So if CMAKE_OSX_ARCHITECTURES is defined by
client, CMakeLists.txt should not force "ppc;i386;x86_64".

CMakeLists.txt

@@ -31,7 +31,9 @@ ADD_DEFINITIONS("-D_CRT_SECURE_NO_DEPRECATE
                  -DALL_INTERIOR_POINTERS")
 
 IF(APPLE)
-    SET(CMAKE_OSX_ARCHITECTURES "ppc;i386;x86_64" CACHE STRING "Build architectures for Mac OS X" FORCE)
+    IF("${CMAKE_OSX_ARCHITECTURES}" STREQUAL "")
+        SET(CMAKE_OSX_ARCHITECTURES "ppc;i386;x86_64" CACHE STRING "Build architectures for Mac OS X" FORCE)
+    ENDIF()
 ENDIF(APPLE)
 
 #LIBATOMIC #TODO