Workaround to build `mirb` with Homebrew's readline on Mac; fix #4537

mrbgems/mruby-bin-mirb/mrbgem.rake

@@ -20,6 +20,15 @@ MRuby::Gem::Specification.new('mruby-bin-mirb') do |spec|
       spec.linker.libraries << 'edit'
     else
       spec.linker.libraries << 'readline'
+      if RUBY_PLATFORM.include?('darwin')
+        # Workaround to build with Homebrew's readline on Mac (#4537)
+        lib_path = spec.build.cc.header_search_paths.find do |include_path|
+          lib_path = File.expand_path("#{include_path}/../lib")
+          break lib_path if File.exist?("#{lib_path}/libreadline.dylib") ||
+                            File.exist?("#{lib_path}/libreadline.a")
+        end
+        spec.linker.library_paths << lib_path if lib_path
+      end
       if spec.build.cc.search_header_path 'curses.h'
         spec.linker.libraries << 'ncurses'
       end