Because of the INTERNAL_tool_NAME variable being a
non-empty string (while it was empty in the previous top-level
invocation), tool.make will include the actual rules to build
the tool; in particular, the internal-tool-all rule, which is
then executed and builds the tool. All variables such as
OBJC_FILES or the library flags are now available directly
in the makefiles, they have already been prepared and preprocessed, so
that the rules in tool.make can just plainly use these
variables naively to perform their job (compiling, installing, or
whatever).