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

There has been svn-bullet.bat for Windows, but not all Linux distributions
provide package managers with BDE support.

There are multiple ways to configure BDE: cmake, premake4 and autogen.sh.
We have been using premake4 for Windows, but cmake is recommended by
bullet/INSTALL, so we use cmake in Linux too.
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.
Pass local zlib's include directory path to clib Makefile

The system could have zlib-devel package installed, but we prefer our
local version.
Merge branch 'clib' into next2
Add library paths for Bullet in GNUMakefile

If there is no system-provided Bullet Dynamics Engine libraries, we must
link the executable with local SVN checkout of BDE.  Also, we want BDE use
double precision for calculations, which may not be the case for system-
provided package.  Configuration for BDE will be done by svn-bullet.sh.
Make vastspace mod build with Makefile for linux gcc

We have not updated the Makefile since we had moved and renamed gltestdll
to mods/vastspace.  Now we'll fix this.
Add much information about building Linux server to README