Add warning about Apple's weird Clang version.
Add warning about Apple's weird Clang version.

MarPiRK/capnproto,mologie/capnproto,modulexcite/capnproto,mrdomino/capnproto,joliss/capnproto,pjulien/capnproto,joliss/capnproto,rcrowder/capnproto,modulexcite/capnproto,ocdtrekkie/capnproto,martindale/capnproto,vladon/capnproto,Fraser999/capnproto,bryce-gibson/capnproto,nagyistoce/capnproto,hntrmrrs/capnproto,bryce-gibson/capnproto,ligzy/capnproto,zarvox/capnproto,ligzy/capnproto,johnkdoe/capnproto,khklau/capnproto,ocdtrekkie/capnproto,mologie/capnproto,maurer/capnproto,rcrowder/capnproto,martindale/capnproto,mrdomino/capnproto,nagyistoce/capnproto,hntrmrrs/capnproto,artillery/capnproto,kamalmarhubi/capnproto,MarPiRK/capnproto,maurer/capnproto,jparyani/capnproto,Fraser999/capnproto,mrdomino/capnproto,mologie/capnproto,artillery/capnproto,khklau/capnproto,bsn069/capnproto,johnkdoe/capnproto,rcrowder/capnproto,kamalmarhubi/capnproto,kamalmarhubi/capnproto,khklau/capnproto,geofft/capnproto,modulexcite/capnproto,geofft/capnproto,ocdtrekkie/capnproto,artillery/capnproto,zarvox/capnproto,zarvox/capnproto,pjulien/capnproto,Fraser999/capnproto,joliss/capnproto,vladon/capnproto,ligzy/capnproto,maurer/capnproto,johnkdoe/capnproto,MarPiRK/capnproto,bryce-gibson/capnproto,nagyistoce/capnproto,a-richardson/capnproto,jparyani/capnproto,geofft/capnproto,a-richardson/capnproto,a-richardson/capnproto,jparyani/capnproto,bsn069/capnproto,hntrmrrs/capnproto,bsn069/capnproto,vladon/capnproto,martindale/capnproto,pjulien/capnproto