add a SYSCONFDIR variable for the configuration directory instead of forcing /etc
revert to upstream. relocatable cvmfs is too much work for little gain
add missing cmake module to find system leveldb
changed config.h to cvmfs_config.h
Merge pull request #167 from sfabbro/master

small fixes to build with system libraries

a couple fixes to build cvmfs from sources with system libraries (packaging for gentoo distro):
    tracer.cc was including a generated file config.h which is not generated while using the *_BUILTIN=OFF
    find_package(leveldb) was assuming a Findleveldb.cmake module not available. this module is a slight modification from another project.