The included zlib should not be used on Linux or OS X, but (depending on
local configuration) it might be needed for crosscompiling a mingw target
from Linux. Now the user can choose whether to use the included zlib or not.
cmake does already support that.
zconf.h.in was taken from the original zlib 1.2.8 package. The generation of
zconf.h was made equivalent to the one generated by cmake.
The new 3rdparty libraries are not added to the autotools build system, so
that they appear in the source package. They cannot be compiled yet with
autotools (although this would make sense for cross compilation with mingw),
but adding configure support for enabling/disabling these like we do for
boost is a bit more work, so I did not do it for now. Nevertheless, the
three new Makefile.am are already prepared for compilation.
