ruby: try using homebrew for SDL in local builds on macOS

ruby/GNUmakefile

@@ -22,10 +22,18 @@ ifeq ($(ruby),)
     ruby += input.quartz #input.carbon
     ifeq ($(sdl2),true)
       macsdl = ../thirdparty/SDL/libSDL2-2.0.0.dylib
-      ares.dylibs += $(macsdl)
       ifeq ($(wildcard $(macsdl)),)
-        $(error Tried to compile ruby for macOS with SDL2 linked, but no SDL2 library was found. Compile it with thirdparty/SDL/build-sdl.sh, or disable SDL by compiling ares with sdl2=false)
+        ifeq ($(local),true)
+          sdl2prefix = $(shell brew --prefix sdl2)
+          ifneq ($(sdl2prefix),)
+            macsdl = $(sdl2prefix)/lib/libSDL2-2.0.0.dylib
+          endif
+        endif
+      endif
+      ifeq ($(wildcard $(macsdl)),)
+        $(error Tried to compile ruby for macOS with SDL2 linked, but no SDL2 library was found. Install it with homebrew, or compile it with thirdparty/SDL/build-sdl.sh, or disable SDL by compiling ares with sdl2=false)
       endif
+      ares.dylibs += $(macsdl)
       ruby += input.sdl
     endif
   else ifeq ($(platform),linux)