GCC does not support strong enum in pre-C++0x mode.

GCC does not support strong enum if -std=c++0x is not used.
Without the strong enum, we will see following error:

  In file included from libcxx/include/ostream:131:0,
                   from libcxx/include/sstream:174,
                   from libcxx/include/complex:247,
                   from cpp03-headers.cpp:11:
  libcxx/include/ios:419:68: error: 'io_errc' is not a class or namespace
  libcxx/include/ios:420:66: error: 'io_errc' is not a class or namespace

To workaround this issue, this commit will define
_LIBCPP_HAS_NO_STRONG_ENUMS when we are compiling with
g++ without c++0x.