for bug #15936.

On some platforms fenv.h may #undef the min/max macros
defined in my_global.h.

Fixed by moving the #include directive for fenv.h from
mysqld.cc to my_global.h before definitions for min/max.

include/my_global.h:
  Moved #include <fenv.h> from mysqld.cc.
sql/mysqld.cc:
  Moved #include <fenv.h> to my_global.h.