Add definition for Ruby 2.7.0

Ruby 2.7.0 has been released.
https://www.ruby-lang.org/en/news/2019/12/25/ruby-2-7-0-released/

add ruby 2.7.0-rc2

Add definition for Ruby 2.7.0-rc1

Ruby 2.7.0-rc1 has been released.
https://www.ruby-lang.org/en/news/2019/12/17/ruby-2-7-0-rc1-released/

Add definition for Ruby 2.7.0-preview3

Ruby 2.7.0-preview3 has been released.
https://www.ruby-lang.org/en/news/2019/11/23/ruby-2-7-0-preview3-released/

And this PR has been verified using a modified checksum.
https://github.com/ruby/www.ruby-lang.org/pull/2284

* The openssl built by ruby-build was not used by TruffleRuby,
  and causes https://github.com/oracle/truffleruby/issues/1818
* Improve error message when openssl from Homebrew is not available.
* Change the definition code so it checks the operating system too.

* See https://github.com/oracle/truffleruby/issues/1818

Add definition for mruby 2.1.0

as an workaround for Ruby bug 16331: https://bugs.ruby-lang.org/issues/16331

Due to this bug, build will fail with FreeBSD's make after #1368.
The bug is already fixed in MRI upstream but GNU make is still required
when building older releases of Ruby. Use GNU make rather than switching
make/gmake depending of Ruby version.

See also: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=241633

Update to OpenSSL 1.1.1d for Ruby 2.5+

Closes #1232

Don’t look for Homebrew openssl

Closes #1353 closes #1360

Fix #1311

https://travis-ci.org/rbenv/ruby-build/builds/607144159

Ref. #1234

Closes #1234

This is for simplicity and usability in scripts.

Fixes #1170

Hopefully this works between BSD and GNU mktemp.

Closes #540