Merge pull request #5318 from shuujii/fix-breakage-where-builds-except-host-dont-refer-mrbc-in-host

Fix breakage where builds except `host` don't refer` mrbc` in `host`

lib/mruby/build.rb

@@ -320,12 +320,16 @@ EOS
       return @mrbcfile if @mrbcfile
 
       gem_name = "mruby-bin-mrbc"
-      gem = @gems[gem_name]
-      gem ||= (host = MRuby.targets["host"]) && host.gems[gem_name]
-      unless gem
-        fail "external mrbc or mruby-bin-mrbc gem in current('#{@name}') or 'host' build is required"
+      if (gem = @gems[gem_name])
+        @mrbcfile = exefile("#{gem.build.build_dir}/bin/mrbc")
+      elsif !host? && (host = MRuby.targets["host"])
+        if (gem = host.gems[gem_name])
+          @mrbcfile = exefile("#{gem.build.build_dir}/bin/mrbc")
+        elsif host.mrbcfile_external?
+          @mrbcfile = host.mrbcfile
+        end
       end
-      @mrbcfile = exefile("#{gem.build.build_dir}/bin/mrbc")
+      @mrbcfile || fail("external mrbc or mruby-bin-mrbc gem in current('#{@name}') or 'host' build is required")
     end
 
     def mrbcfile=(path)