On most UNIX systems, it should not be necessary to set any of these variables, since the zlib and libbz2 headers and binaries are already installed in locations where they will be found automatically. On Windows, the zlib and bzip2 filters are disabled by default; to enable support for these filters, you will need to set variables indicating the location of the source files, if building from the source, or the location of the headers and binaries, if using pre-built binaries.
++ NOTE: When building the bzip2 library from the source as a DLL on Windows, a small change needs to be made in the bzip2 source files. As of version 1.0.4 of bzip2, a libbz2.DEF file is used to indicate the exported functions for Windows builds. This file also contains a line with a LIBRARY directive indicating the standard library name of LIBBZ2. Since the Boost.Build jamfiles prefer to build the bzip2 library with a boost_bzip2 name, there is a conflict with this LIBRARY directive. To allow the boost_iostreams DLL to successfully find the bzip2 DLL with its boost_bzip2 name, the LIBRARY directive line in libbz2.DEF should be commented out by placing a semicolon at the beginning of the line as follows: +
++ ; LIBRARY LIBBZ2 +
++ With this change to the DEF file, Boost.Build should be able to successfully generate a DLL version of the bzip2 library from its sources. +