cmake script does not check for liblog

[sarfata]

On Ubuntu 14.04, I got the following error when compiling with make:

/usr/include/gnuradio/logger.h:116:31: fatal error: log4cpp/Category.hh: No such file or directory
 #include <log4cpp/Category.hh>
                               ^
compilation terminated.

I fixed it with sudo apt-get install liblog4cpp5-dev but I think the cmake script should check for this library at configure time.