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(-)

Rdoc [build] of `master` did not have this method but the reference manual [did].

[build]: https://docs.ruby-lang.org/en/master/RbConfig.html
[did]: https://docs.ruby-lang.org/ja/master/class/RbConfig.html#S_RUBY

This commit adds the ABI version as build metadata to
RUBY_LIB_VERSION. This will ensure that gems are installed in a path
with the ABI version.

Header file include/ruby/internal/abi.h contains RUBY_ABI_VERSION which
is the ABI version. This value should be bumped whenever an ABI
incompatible change is introduced.

When loading dynamic libraries, Ruby will compare its own
`ruby_abi_version` and the `ruby_abi_version` of the loaded library. If
these two values don't match it will raise a `LoadError`. This feature
can also be turned off by setting the environment variable
`RUBY_RUBY_ABI_CHECK=0`.

This feature will prevent cases where previously installed native gems
fail in unexpected ways due to incompatibility of changes in header
files. This will force the developer to recompile their gems to use the
same header files as the built Ruby.

In Ruby, the ABI version is exposed through
`RbConfig::CONFIG["ruby_abi_version"]`.

Since these regexps are used at build/installation, they are not
vulnerabilities.

c.f. https://opensource.apple.com/source/ruby/ruby-145.40.1/patches/tool_mkconfig.rb.diff

* moved to "includedir" from "oldincludedir".
* honor "$(prefix)/include" if exists, otherwise fallback to `xcrun`.

Means the platform with the actual CPU info on universal binary.
The CPU in "arch" is fixed as "universal" to refer the header and
library paths.