The reason  for the bug is that :
-  we use system checks in cmake/os/mysql_release.cmake 
-  we include cmake/os/mysql_release.cmake using CMAKE_USER_MAKE_RULES_OVERRIDE 
-  this  (having system checks based on TRY_COMPILE inside file pointed by 
   CMAKE_USER_MAKE_RULES_OVERRIDE  does not work with cmake 2.8.3,
   and  according to Kitware was never meant to work, it just happened to work by accident
   until 2.8.2 release (though, it seems not to work wiith 2.6.0 either)

Related CMake bug discussing the situation:
http://public.kitware.com/Bug/view.php?id=11469  

The fix is to use INCLUDE instead of CMAKE_USER_MAKE_RULES_OVERRIDE as suggested
by Kitware. The downside is that compile flags are not in cache, but this is  pure cosmetics.
The functionality is the same,  flags are used for compiling are correct using INCLUDE.