fix mingw64 installer

Makefile

@@ -561,16 +561,19 @@ ifeq ($(PLATFORM),mingw32)
                       $(LIBSDIR)/win32/libSDL.dll.a
     RENDERER_LIBS += $(LIBSDIR)/win32/libSDLmain.a \
                       $(LIBSDIR)/win32/libSDL.dll.a
+    SDLDLL=SDL.dll
     else
     CLIENT_LIBS += $(LIBSDIR)/win64/libSDLmain.a \
                       $(LIBSDIR)/win64/libSDL64.dll.a
     RENDERER_LIBS += $(LIBSDIR)/win64/libSDLmain.a \
                       $(LIBSDIR)/win64/libSDL64.dll.a
+    SDLDLL=SDL64.dll
     endif
   else
     CLIENT_CFLAGS += $(SDL_CFLAGS)
     CLIENT_LIBS += $(SDL_LIBS)
     RENDERER_LIBS += $(SDL_LIBS)
+    SDLDLL=SDL.dll
   endif
 
   BUILD_CLIENT_SMP = 0
@@ -2493,6 +2496,7 @@ distclean: clean toolsclean
 installer: release
 ifeq ($(PLATFORM),mingw32)
 	@$(MAKE) VERSION=$(VERSION) -C $(NSISDIR) V=$(V) \
+		SDLDLL=$(SDLDLL) \
 		USE_RENDERER_DLOPEN=$(USE_RENDERER_DLOPEN) \
 		USE_OPENAL_DLOPEN=$(USE_OPENAL_DLOPEN) \
 		USE_CURL_DLOPEN=$(USE_CURL_DLOPEN) \

misc/nsis/Makefile

@@ -29,12 +29,7 @@ ifndef USE_INTERNAL_JPEG
 USE_INTERNAL_JPEG=1
 endif
 
-
-ifeq ($(ARCH),x64)
-  SDLDLL=SDL64.dll
-else
-  SDLDLL=SDL.dll
-endif
+SDLDLL=SDL.dll
 
 DEFINES=
 ifeq ($(USE_RENDERER_DLOPEN),1)