SimGear change. It changes all the SG_xxxx to be the 'real' includes, and gets
rid of many #ifdef SG_HAVE_STD_INCLUDES. As an added bonus, rather than
replacing 'SG_USING_NAMESPACE(std)' with 'using namespace std', I just fixed
the small number of places to use std:: explicitly. So we're no longer polluting
the global namespace with the entire contents of std, in many cases.
There is one more 'mechanical' change to come - getting rid of SG_USING_STD(X),
but I want to keep that separate from everything else. (There's another
mechnical change, replacing <math.h> with <cmath> and so on *everywhere*, but
one step at a time)
