[tools] Don't build legacy stuff unless legacy is enabled. (#15773)

tools/Makefile

@@ -3,14 +3,32 @@ TOP=..
 include $(TOP)/Make.config
 
 ifdef INCLUDE_MAC
+ifdef INCLUDE_XAMARIN_LEGACY
 SUBDIRS+=mmp
 endif
+endif
 
-SUBDIRS+=mtouch xibuild mlaunch siminstaller nnyeah
+ifdef INCLUDE_XAMARIN_LEGACY
+SUBDIRS += mtouch
+endif
 
+ifdef INCLUDE_XAMARIN_LEGACY
+SUBDIRS += xibuild
+endif
+
+SUBDIRS+=mlaunch siminstaller
+
+ifdef ENABLE_DOTNET
+ifdef INCLUDE_XAMARIN_LEGACY
+SUBDIRS += nnyeah
+endif
+endif
+
+ifdef INCLUDE_XAMARIN_LEGACY
 ifdef ENABLE_INSTALL_SOURCE
 SUBDIRS += install-source
 endif
+endif
 
 ifdef ENABLE_DOTNET
 SUBDIRS += dotnet-linker