build: prevent SDL2 from polluting global cflags and extralibs

Remove the SDL_main define from the global cflags but not from the
ffplay cflags, and the -mwindows linker option from extralibs instead
of overriding it with the addition of -mconsole.

Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

ffbuild/library.mak

@@ -16,7 +16,6 @@ all-$(CONFIG_SHARED): $(SUBDIR)$(SLIBNAME) $(SUBDIR)lib$(FULLNAME).pc
 
 LIBOBJS := $(OBJS) $(SUBDIR)%.h.o $(TESTOBJS)
 $(LIBOBJS) $(LIBOBJS:.o=.s) $(LIBOBJS:.o=.i):   CPPFLAGS += -DHAVE_AV_CONFIG_H
-$(TESTOBJS) $(TESTOBJS:.o=.i): CFLAGS += -Umain
 
 $(SUBDIR)$(LIBNAME): $(OBJS)
 	$(RM) $@