PROTON-705: Work with older versions of CMake too

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1628503 13f79535-47bb-0310-9956-ffa450edef68
NO-JIRA: If we didn't build the python bindings then we can't run python tests

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1628509 13f79535-47bb-0310-9956-ffa450edef68
NO-JIRA: Fix broken CMake initialisation of BUILD_WITH_CXX