Include stdbool.h without checking with autoconf
As reported in <https://bugs.ruby-lang.org/issues/21340>, older autoconf
have an AC_HEADER_STDBOOL that's incompatible with C23. Autoconf 2.72
fixed the macro, but also mentions that it's obsolescent since all
current compilers have this header.
Since we require C99 [1] and VS 2015 [2], we might actually be able take
that suggestion and include stdbool.h without a check. I want to try
this on rubyci.org and will revert if this cause any issues. Not
touching AC_HEADER_STDBOOL in configure.ac for now.
[1]: https://bugs.ruby-lang.org/issues/15347
[2]: https://bugs.ruby-lang.org/issues/19982
Related issues
Bug #21340: Bump autoconf version to properly handle C23 bool/stdbool defines
merge revision(s) 2297afda7ff3926c51fea700dfbf0f0eb4fea1e5: [Backport #21340]