cpp: BUILD_ASSERT() uses static_assert() iff C++11

The #if to use static_assert() for BUILD_ASSERT() only checks
whether compiler is in C++ mode, but does not check what
C++ standard is being used. Since static_assert() is only
valid since C++11, for compilers not supporting C++11 (XCC...),
this will result in error. So add the correct #if to also
check for C++ standard being used.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>