Make the Makefile use local sources for Bullet and zlib

Previously, we have been using system-provided library headers to complie
the libraries, but not all systems have them or have package managers
available to install them easily.
Make UnZip.c do not depend on compiler int sizes

We also use alignment control directives for VC and gcc.
Add a test program source to dump UnZip variable sizes
clib: Makefile accumulate CFLAGS given as env var
Pass local zlib's include directory path to clib Makefile

The system could have zlib-devel package installed, but we prefer our
local version.