C23 compliance

[orlitzky]

Original report: https://bugs.gentoo.org/944356

Guava defines a bool type in the obvious way, but the C23 standard now also defines bool. This leads to a build failure in -std=c23 mode that will happen by default in the next version of GCC, so it would be good to either append std=c17 to the build, or #ifdef around the bool type before then.

[osj1961]

Followed the suggestion for #ifdef-ing the bool definition:
In 3dc3b1a

[orlitzky]

That did the trick, thanks!