Turn off C++ API in CMake script by default

Turn off C++ API in CMake script by default

Issue #281 (bdwgc).

This is to match the behavior of configure script.
Now, -Denable_cplusplus=ON should be passed to cmake to enable C++ API.

* CMakeLists.txt (enable_cplusplus): Add option (off by default);
remove TODO.
* CMakeLists.txt (SRC): Add gc_cpp.cc only if enable_cplusplus.
* tests/CMakeLists.txt (leak_test.c, test.c): Set CXX langunage
property only if enable_cplusplus.
* tests/CMakeLists.txt [enable_cplusplus]: Add TODO to add test_cpp as
a test.