Apple uses different __clang_major__ version numbers than upstream,
 so our test did not work.
This caused compilation failures with, e.g., XCode 10.1, which
 reports __clang_major__ as 10 despite being forked from upstream's
 7.0 branch.

Fixes #2369

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>