Support for Clang with "-fgnuc-version=0". Previously, the code assumed that Clang pretends to be GCC and defines the __GNUC__ macro, which might not be the case if the "-fgnuc-version=0" flag is used. Fix is to explicitly check for __clang__ as well. In practice, with this change ngx_cpuinfo() is now available on Linux with "-fgnuc-version=0".