Added missing cmake link to -ldl

Missing link for dlsym. Added link to library for cmake.
System:
Debian 7.4
Cmake states:

Installation Summary
--------------------
* Install Directory            : /usr/local
* Build libgroove              : yes
* Build libgrooveplayer        : yes
* Build libgrooveloudness      : yes

Bundled Dependencies
--------------------
* SDL2                         : using system library
* libav                        : ready to build
* libebur128                   : using system library

System Dependencies
-------------------
* C99 Compiler                 : OK
* threads                      : OK
* SDL2                         : OK
* ebur128                      : OK
* libavformat                  : not found - will use bundled version
* libavcodec                   : not found - will use bundled version
* libavfilter                  : not found - will use bundled version
* libavutil                    : not found - will use bundled version
* yasm                         : OK
* bzip2                        : OK
* mp3lame                      : OK
* zlib                         : OK