[GiNaC-list] Linking dl libraries is missing in ginac.pc

Hi, 5745deb8 introduced linking with dl libraries, but it was not added to the .pc files. Therefore list of link flags given by pkg-config is incomplete and fails if `-ldl` is not added manually. I have attached a patch which uses CMAKE_DL_LIBS variable ( http://www.cmake.org/cmake/help/cmake2.6docs.html#variable:CMAKE_DL_LIBS) Thanks, Isuru Fernando