Build gc as a shared library by default (CMake)

Build gc as a shared library by default (CMake)

Issue #281 (bdwgc).

* CMakeLists.txt (BUILD_SHARED_LIBS): Document option and set default to ON.
* CMakeLists.txt: Remove commented out ADD_LIBRARY for atomic_ops.
* CMakeLists.txt [BUILD_SHARED_LIBS] (SRC): Override to extra/gc.c.
* CMakeLists.txt [BUILD_SHARED_LIBS] (GC_DLL): Define macro.
* CMakeLists.txt [!BUILD_SHARED_LIBS] (GC_NOT_DLL): Define macro not only
for tests but also for gc library.
* CMakeLists.txt (gc): Remove STATIC in add_library specification.