include cmake files in tarball

This includes some cmake files so as to allow to use cmake with the dowloaded tarball.

BUT, first there's another problem to fix with 'make dist' (I just did autogen.sh, ../configure):

make[3]: Entering directory '/home/xantares/projects/libzmq/build/doc'
make[3]: *** No rule to make target 'zmq_bind.3', needed by 'dist-hook'.  Stop.
make[3]: Leaving directory '/home/xantares/projects/libzmq/build/doc'
Makefile:444: recipe for target 'distdir' failed

x.