OSX: Don't set a default MACOSX_DEPLOYMENT_TARGET.

doc/install.html

@@ -212,8 +212,8 @@ You can add an extra prefix to the search paths by appending the
 make PREFIX=/home/myself/lj2
 </pre>
 <p>
-Note for OSX: if the <tt>MACOSX_DEPLOYMENT_TARGET</tt> environment
-variable is not set, then it's forced to <tt>10.4</tt>.
+Note for OSX: you must set the <tt>MACOSX_DEPLOYMENT_TARGET</tt>
+environment variable to a value supported by your toolchain.
 </p>
 <h3>Installing LuaJIT</h3>
 <p>

src/Makefile

@@ -304,7 +304,7 @@ 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
+    $(error missing: export MACOSX_DEPLOYMENT_TARGET=XX.YY)
   endif
   TARGET_STRIP+= -x
   TARGET_XSHLDFLAGS= -dynamiclib -single_module -undefined dynamic_lookup -fPIC