linux: Add svn-bullet.sh to checkout BulletDynamics Engine on Linux

There has been svn-bullet.bat for Windows, but not all Linux distributions
provide package managers with BDE support.
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.
clib: Make UnZip.c do not depend on compiler int sizes

We also use alignment control directives for VC and gcc.
clib: Add a test program source to dump UnZip variable sizes
clib: Makefile accumulate CFLAGS given as env var
Merge branch 'clib' into next2
Pass local zlib's include directory path to clib Makefile

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