Refactoring: move GUID operations out of Joystick

- SDL_JoystickGUID -> SDL_GUID (though we retain a type alias)
- Operations for GUID <-> String ops are now in
  src/SDL_guid.c and include/SDL_guid.h
- The corresponding Joystick operations delegate to SDL_guid.c
- Added test/testguid.c

test/Makefile.in

@@ -40,6 +40,7 @@ TARGETS = \
 	testgamecontroller$(EXE) \
 	testgeometry$(EXE) \
 	testgesture$(EXE) \
+	testguid$(EXE) \
 	testhaptic$(EXE) \
 	testhittesting$(EXE) \
 	testhotplug$(EXE) \
@@ -201,7 +202,7 @@ testgeometry$(EXE): $(srcdir)/testgeometry.c $(srcdir)/testutils.c
 
 testgesture$(EXE): $(srcdir)/testgesture.c
 	$(CC) -o $@ $^ $(CFLAGS) $(LIBS) @MATHLIB@
- 
+
 testgl2$(EXE): $(srcdir)/testgl2.c
 	$(CC) -o $@ $^ $(CFLAGS) $(LIBS) @MATHLIB@
 
@@ -214,6 +215,9 @@ testgles2$(EXE): $(srcdir)/testgles2.c
 testgles2_sdf$(EXE): $(srcdir)/testgles2_sdf.c $(srcdir)/testutils.c
 	$(CC) -o $@ $^ $(CFLAGS) $(LIBS) @MATHLIB@
 
+testguid$(EXE): $(srcdir)/testguid.c
+	$(CC) -o $@ $^ $(CFLAGS) $(LIBS)
+
 testhaptic$(EXE): $(srcdir)/testhaptic.c
 	$(CC) -o $@ $^ $(CFLAGS) $(LIBS)