Later commits will add more information, but this tests the
conditional compilation and build setup.

One way to do conditional compilation: determine this
early, at LLVM configure/CMake time. Configure will
fill in the template of SZTargets.def.in to get
a SZTargets.def file.

LLVM change:
https://codereview.chromium.org/1084753002/

NaCl change:
https://codereview.chromium.org/1082953002/

I suppose an alternative is to fill in the .def file via
-D flags in CXXFLAGS.

For conditional lit testing, pnacl-sz dumps the attributes
when given the --build-atts so we just build on top of that.
We do that instead of go the LLVM way of filling in a
lit.site.cfg.in -> lit.site.cfg at configure/CMake time.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1075363002