Include git commit ID in configuration summary

[program--]

This PR adds the repo's git commit ID to the CMake configuration summary output. This will also include it when calling ngen --info.

Example:

NGen version: 0.1.0
-- Build configuration summary:
--   Generator: Ninja
--   Build type: Debug
--   System: Linux
--   C Compiler: /usr/bin/clang
--   C Flags: -Wpessimizing-move
--   CXX Compiler: /usr/bin/clang++
--   CXX Flags: -Wpessimizing-move -pedantic-errors
--   Git Commit ID: 2ffedf8ecb3d6311be65d8c7c8f99c2fecf7ee07
--   Flags:
...

Additions

• Adds NGEN_GIT_COMMIT CMake variable.

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️

[PhilMiller]

I see the print in configuration. How does it end up in ngen --info? Given that you show runtime output, with strings that don't exist in my repo, I'm guessing some code didn't get committed

[PhilMiller]

Not relevant to this PR, and really an edge case in general, but I think this condition may not work in the case of a git 'worktree', which just leaves some sort of link or reference in the working source directory to the actual repository.

[program--]

I see the print in configuration. How does it end up in ngen --info? Given that you show runtime output, with strings that don't exist in my repo, I'm guessing some code didn't get committed

The ngen_multiline_message and ngen_dependent_multiline_message CMake functions store their string arguments into a separate CMake variable NGEN_CONF_SUMMARY as a side effect. When NGenConfig.h is configured, that variable is outputted as a string here:

ngen/include/NGenConfig.h.in

Line 46 in 2ffedf8

static constexpr const char* build_summary = R"(@NGEN_CONF_SUMMARY@)";

So for every ngen build, the exact build summary is embedded into it via that header

[PhilMiller]

Ah, that was a depth of cleverness that I hadn't realized. Excellent.