add fix for older cmake versions
add fix for older cmake versions

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