Allow disabling libXML in Clang's CMake build

We used this before the Precise upgrade for the same purpose I'm
reintroducing it; the sysroot we are building in lacks libXML but
Clang's CMake build finds it on the host system.

R=stichnot@google.com, bradnelson@chromium.org
BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4394

Review-Url: https://codereview.chromium.org/2769743004 .