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/structure/commands.hxx

@@ -13,7 +13,7 @@
 
 #include <simgear/compiler.h>
 
-#include STL_STRING
+#include <string>
 #include <map>
 #include <vector>
 

simgear/structure/exception.hxx

@@ -11,7 +11,7 @@
 #define __SIMGEAR_MISC_EXCEPTION_HXX 1
 
 #include <simgear/compiler.h>
-#include STL_STRING
+#include <string>
 
 SG_USING_STD(string);
 

simgear/structure/subsystem_mgr.hxx

@@ -31,7 +31,7 @@
 #  include <float.h>
 #endif
 
-#include STL_STRING
+#include <string>
 SG_USING_STD(string);
 
 #include <vector>