Don't rely on __GXX_RTTI being defined with g++ < 4.3.

Don't rely on __GXX_RTTI being defined with g++ < 4.3.

__GXX_RTTI is only defined since g++ 4.3.2 and so we can't rely on its absence
as indicating the use of -fno-rtti switch with the older versions of the
compiler, only do this for g++ 4.3+.

Also define wxNO_RTTI from configure if --enable-no_rtti was used as we can't
always detect it automatically.

Closes #11955.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63995 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775