mkconfig: Map `includedir` only for system ruby

	Only when installing to the system path on macOS, prepend '$(SDKROOT)'
	and remap `includedir`.
	Fix https://github.com/rbenv/ruby-build/discussions/2123
	---
	 test/mkmf/test_config.rb | 4 ++--
	 test/test_rbconfig.rb    | 9 ---------
	 tool/mkconfig.rb         | 4 +++-
	 3 files changed, 5 insertions(+), 12 deletions(-)