Don't require copying the SDL.framework into the bundle

When using MacPorts (and likely Homebrew), this isn't how this works.

src/syncterm/GNUmakefile

@@ -124,8 +124,8 @@ ifeq ($(os),darwin)
 	${QUIET}${INSTALL_DATA} Info.plist ${EXEODIR}/SyncTERM.app/Contents
 	${QUIET}${INSTALL_DATA} SyncTERM.icns ${EXEODIR}/SyncTERM.app/Contents/Resources
 	-${QUIET}rm -rf ${EXEODIR}/SyncTERM.app/Contents/Frameworks/SDL2.framework
-	${QUIET}cp -R ${SDL_FRAMEWORK_PATH}/SDL2.framework ${EXEODIR}/SyncTERM.app/Contents/Frameworks
-	${QUIET}rm -rf ${EXEODIR}/SyncTERM.app/Contents/Frameworks/SDL2.framework/Headers/*
+	-${QUIET}cp -R ${SDL_FRAMEWORK_PATH}/SDL2.framework ${EXEODIR}/SyncTERM.app/Contents/Frameworks
+	-${QUIET}rm -rf ${EXEODIR}/SyncTERM.app/Contents/Frameworks/SDL2.framework/Headers/*
 endif
 
 syncterm.man: syncterm.man.in