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)

simgear/threads/SGQueue.hxx

@@ -3,12 +3,7 @@
 
 #include <simgear/compiler.h>
 
-#if defined ( SG_HAVE_STD_INCLUDES )
-#  include <cassert>
-#else
-#  include <assert.h>
-#endif
-
+#include <cassert>
 #include <queue>
 #include "SGThread.hxx"
 #include "SGGuard.hxx"

simgear/threads/SGThread.hxx

@@ -26,13 +26,8 @@
 #include <simgear/compiler.h>
 
 #include <pthread.h>
-#if defined ( SG_HAVE_STD_INCLUDES )
-#  include <cassert>
-#  include <cerrno>
-#else
-#  include <assert.h>
-#  include <sys/errno.h>
-#endif
+#include <cassert>
+#include <cerrno>
 
 class SGThread;