Do not include 'new' standard header from gc_cpp.h by default

Do not include 'new' standard header from gc_cpp.h by default
(fix of commit cb1194d17)

* gc_cpp.cc: Include gc.h (before "new") and "new" standard header
(before gc_cpp.h).
* gc_cpp.cc (GC_ALLOCATOR_THROW_OR_ABORT): New macro (the same
definition as in gc_allocator.h).
* gc_cpp.cc (GC_throw_bad_alloc): New API function definition.
* gc_cpp.cc [!GC_NEW_DELETE_THROW_NOT_NEEDED]
(GC_NEW_DELETE_NEED_THROW): Do not define if _MSC_VER or __DMC__.
* gc_cpp.cc [!_MSC_VER && !__DMC__] (new, new[]): Replace
GC_OP_NEW_OOM_CHECK(obj) to if(!obj)GC_ALLOCATOR_THROW_OR_ABORT().
* gc_cpp.h: Include "new" standard header only if GC_INCLUDE_NEW
and !GC_NEW_ABORTS_ON_OOM and !_LIBCPP_NO_EXCEPTIONS.
* gc_cpp.h [!GC_NEW_ABORTS_ON_OOM && !_LIBCPP_NO_EXCEPTIONS
&& !GC_INCLUDE_NEW] (GC_throw_bad_alloc): Declare API function.
* gc_cpp.h [!GC_NEW_ABORTS_ON_OOM && !_LIBCPP_NO_EXCEPTIONS
&& !GC_INCLUDE_NEW] (GC_OP_NEW_OOM_CHECK): Call GC_throw_bad_alloc()
instead of throw std::bad_alloc; do not use do-while(0) (to eliminate
VC++ warning that the expression is always false).