locale: Implemented SDL_GetPreferredLocales().

This was something I proposed a long time ago, Sylvain Becker did
additional work on it, then back to me.

Fixes Bugzilla #2131.

test/Makefile.in

@@ -37,6 +37,7 @@ TARGETS = \
 	testjoystick$(EXE) \
 	testkeys$(EXE) \
 	testloadso$(EXE) \
+	testlocale$(EXE) \
 	testlock$(EXE) \
 	testmessage$(EXE) \
 	testmultiaudio$(EXE) \
@@ -303,6 +304,10 @@ controllermap$(EXE): $(srcdir)/controllermap.c
 testvulkan$(EXE): $(srcdir)/testvulkan.c
 	$(CC) -o $@ $^ $(CFLAGS) $(LIBS)
 
+testlocale$(EXE): $(srcdir)/testlocale.c
+	$(CC) -o $@ $? $(CFLAGS) $(LIBS)
+
+
 
 clean:
 	rm -f $(TARGETS)