Reduce compiler.h to almost nothing (but it's worth keeping around I think, for

Reduce compiler.h to almost nothing (but it's worth keeping around I think, for
the MSVC and MipsPro warning stuff).

As a result of this patch, simgear/sg_traits.h can be deleted. So can SGCMath.h,
but I'll do that separately.

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 mechnica
l change, replacing <math.h> with <cmath> and so on *everywhere*, but one step a
t a time)