cmake: XercesIntTypes: Add a <cstdint> functional check

Header presence isn't always sufficient; if the headers is
present but the compiler is in C++98 mode it will error out.


git-svn-id: https://svn.apache.org/repos/asf/xerces/c/trunk@1798747 13f79535-47bb-0310-9956-ffa450edef68