Prevent accidental liblzma dependency

When building from Cygwin with the mingw64-{i686,x86_64}-xz packages
installed, libtiff and libxml2 were autodetecting the packaged liblzma
and linking against it.  Prevent this.

http://lists.andrew.cmu.edu/pipermail/openslide-users/2014-May/000798.html
Explicitly disable additional unneeded dependencies

in case they happen to exist in the cross-compilation environment.
Skip building TurboJPEG