Cherry-pick fix for LuaJIT/LuaJIT#484 to fix builds on MacOS Mojave.

Bump minimum MACOSX_DEPLOYMENT_TARGET to 10.6

The macOS SDK for 10.4 has been deprecated as of 10.14, bump to 10.6 so that LuaJIT continues to build out of the box.

Fixes LuaJIT#484.

third_party/luajit/luajit/src/Makefile

@@ -310,7 +310,7 @@ ifeq (,$(shell $(TARGET_CC) -o /dev/null -c -x c /dev/null -fno-stack-protector
 endif
 ifeq (Darwin,$(TARGET_SYS))
   ifeq (,$(MACOSX_DEPLOYMENT_TARGET))
-    export MACOSX_DEPLOYMENT_TARGET=10.4
+    export MACOSX_DEPLOYMENT_TARGET=10.6
   endif
   TARGET_STRIP+= -x
   TARGET_AR+= 2>/dev/null