The '-ll' (two lower-case 'ells') is a linker option that says 'link with "libl.so"', where libl.so is the support library required for programs built with (f)lex. The error message is telling you that the linker cannot find that file. You can find it, and add the accordant '-L /path/to/flex/lib' option to the compiler or linker commandline.
For details, man ld.
It is possible that the library is not installed (although that would seem strange, if flex is installed), and you will have to use a package manager to install it. In that case, I would anticipate that it will be installed in a place normally found by ld.
Last edited by theNbomr; 03-11-2013 at 10:58 AM.