Suppress gcc 15 unterminated-string-initialization warnings

	Separate `__has_attribute` from `defined(__has_attribute)`

	Fix Visual C warnings:
	```
	regenc.h(121): warning C4067: unexpected tokens following preprocessor directive - expected a newline
	```

	Cast up `int` instruction code to `VALUE`

	Fix Visual C warnings:
	```
	iseq.c(3793): warning C4312: 'type cast': conversion from 'int' to 'void *' of greater size
	iseq.c(3794): warning C4312: 'type cast': conversion from 'int' to 'void *' of greater size
	```

	Do not let files depend on a phony target

	Detect `clock_gettime` and `clock_getres` for winpthreads

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], should be able take
that suggestion and include stdbool.h without a check. This worked
fine on rubyci.org.

[Backport #21340]

[1]: https://bugs.ruby-lang.org/issues/15347
[2]: https://bugs.ruby-lang.org/issues/19982

Backport [Bug #21286]

And finally deprecated at C++-17.
Patched by jprokop (Jarek Prokop).

The macro MAYBE_UNUSED, prepared by ./configure, may not be defined in
some environments such as Oracle Developer Studio 12.5 on Solaris 10.

This fixes [Bug #20963]

Thanks, nobu!

`_umul128` is specific to x86_64 platform, see higher words by
`__umulh` on arm64.

It is not "in bytes" for wide char literal.