Optionally put gc_cpp symbols into 'boehmgc' namespace

Optionally put gc_cpp symbols into 'boehmgc' namespace

* include/gc_cpp.h (GC_NAMESPACE): Recognize new macro.
* include/gc_cpp.h (GC_NS_QUALIFY): New macro definition (prefixing
argument with "boehmgc" namespace if GC_NAMESPACE).
* include/gc_cpp.h (GCPlacement, gc, gc_cleanup, GCCleanUpFunc,
gc::operator new, gc::operator delete, gc::operator new[],
gc::operator delete[], gc_cleanup::~gc_cleanup, gc_cleanup::cleanup):
Put into "boehmgc" namespace if GC_NAMESPACE.
* include/gc_cpp.h (operator new, operator delete, operator new[]):
Wrap GCPlacement, GCCleanUpFunc, UseGC, PointerFreeGC symbols into
GC_NS_QUALIFY.
* tests/test_cpp.cc (USE_GC, main): Wrap UseGC, GC, NoGC symbols into
GC_NS_QUALIFY.
* tests/test_cpp.cc (class B, class C, class D, class E): Wrap gc,
gc_cleanup class names into GC_NS_QUALIFY.