Build libcord by CMake script

Build libcord by CMake script

Issue #281 (bdwgc).

* CMakeLists.txt [build_cord] (CORD_SRC): Set variable.
* CMakeLists.txt [build_cord] (cord): Specify add_library and
target_link_libraries; remove TODO.
* CMakeLists.txt [install_headers && build_cord] (cord/cord.h,
cord/cord_pos.h, cord/ec.h): Specify install.
* CMakeLists.txt [build_tests && build_cord] (cordtest, de): Do not
compile cord/cordbscs.c, cord/cordprnt.c, cord/cordxtra.c; add cord to
target_link_libraries specification.