gzstream temporarily disabled

[Jochem Snuverink]

commit f2e53a3 disables the gzstream code (used by BDSBunchUserFile) as users (@hector_garcia_morales) reported linker problems (on Ubuntu 14, gcc version?).

Querying the bdsim executable shows that there are undefined symbols:

nm bdsim |grep gzclose
                 U gzclose

The src-external/CMakeLists.txt needs to be improved so that it finds the zlib library and links explicitly to it.

Also we should have a working test example.

[Laurie Nevay]

The CMakelists.txt has been expanded so that the gzstream library we create is linked against the Geant4 version of zlib or if that's not available the system version. If that's not available, gzstream is disabled and the code commented out by preprocessor defines. I was able to compile this successfully on ubuntu14.1 LTS, SL6 and mac ox 10.11 without problem so I'm closing this issue.