Log Message:
s/USE_CXX/USE_GCC_SHLIB/ - there are more shared libraries than just
libstdc++ in gcc3.

when defined, USE_GCC_SHLIB ensures that the correct rpath is passed
to the linker, and a full dependency on the compiler package is
registered.

packages which define USE_GCC_SHLIB should not include
mk/gcc.buildlink2.mk (or gcc{,3}/buildlink2.mk) as it is handled
automatically.