The makefile assumes a _snackmodule.so target, which is not correct on Mac OSX. Why not use $(SOEXT) here?
I'm not familiar with building on OS X, but I'll be happy to include patches fixing the support.
Macports already has the proper patch https://trac.macports.org/browser/trunk/dports/devel/libnewt/files/patch-Makefile.in.diff https://trac.macports.org/browser/trunk/dports/devel/libnewt/files/patch-configure.ac.diff
The Makefile now uses SOEXT instead of so and can be overridden on the commandline. I think the other changes in the patches from comment #2 need more work to be accepted, e.g. the linker flags shouldn't be changed on non-Darwin systems. https://git.fedorahosted.org/cgit/newt.git/commit/?id=673e30095776571c4e59ad44d93ed040828ea43f
This is in newt-0.52.16.