Add cxx_cmd which parallels cc_cmd

[flavorjones]

Add a cxx_cmd. Use it for configuring CMake, removing the logic added in #130 for host-specific binary detection.

Related to #141.

[flavorjones]

@stanhu Do you want to give this branch a try on your projects before I merge, to make sure I'm not breaking anything?

[stanhu]

@flavorjones Sure, I'm running a test with the re2 gem now.

[stanhu]

I tested locally, and the CI runs in mudge/re2#151 look good. The failures are due to not having mini_portile2 available as a gem. We can re-test with a release.

[flavorjones]

@stanhu Thanks for testing! I'll get an RC out this week.

[flavorjones]

@stanhu I just released v2.8.7.rc1. If you're able to run that through re2 CI to confirm it's OK for you then I will cut a final release.

Side note: I'm going to try to set up a downstream CI workflow to test re2, sqlite3, and nokogiri.

Edit: that PR is #146.

[stanhu]

Thanks! I created mudge/re2#153 to test.

Side note: I'm going to try to set up a downstream CI workflow to test re2, sqlite3, and nokogiri.

Awesome, great idea. FYI @mudge

[stanhu]

v2.8.7.rc1 seems to work fine.

[flavorjones]

@stanhu Thank you! The downstream integration tests are live, FYI: https://github.com/flavorjones/mini_portile/actions/runs/9324948020

[flavorjones]

v2.8.7 final is out. Thank you again!