Explicitly guard gcc's code path using BOOST_CLANG
.
Currently, the version check (__GNUC__ == 4) && (__GNUC_MINOR__ >= 3)
is enough to guard gcc's code path from clang, since clang pretends to be gcc-4.2.1.
But, it seems that the clang's (pretended) gcc version will be changed to a more recent one in the near future. So we need the explicit guard.