Added top-level comment with cmake instructions
Fixed unused search for pkgconfig

This was used in the log4cxx cmake configuration but only result
in a misleading error message.
Proper search for dependencies

 * zlib and librt now searched in a more appropriate way.
 * Tidied up the layout of the src/CMakeLists.txt file and removed unused
   comments.
Fixed the Boost library search

The cmake supplied FindBoost.cmake is broken (at least in cmake 2.8) and
the system libraries are always returned even when BOOST_ROOT is specifically
set and system search paths disabled. This commit provide a work-around for
this problem where the BOOST_ROOT path (if set) is searched for boost libs.