libxenguest: correct order of zlib linker options to avoid link errors

libxenguest: correct order of zlib linker options to avoid link errors

The order of objects and linked libraries is important with recent
toolchain when --as-needed is used.

Introduce new variable COMPRESSION_LIBS and place it at the end of cc
commandline options.  Without this change, symbols in libbzip2 are not
found when building with openSuSE 11.2 and newer.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>