I was not aware of this because I use clang these days.

We don't want to document e.g. M_PI, which is an ISO C thing.

This function should be always available, as POSIX-compliant or
Windows platform are required since 1.9.  Also the code in this
file is MT-unsafe.

To fix build failures.

Devs do not love "3".  The only exception is RUBY3_KEYWORDS in parse.y,
which seems unrelated to our interests.

This shall fix compile errors.

According to MSVC manual (*1), cl.exe can skip including a header file
when that:

- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.

GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).

Sun C lacked #pragma once for a looong time.  Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.

This changeset modifies header files so that each of them include
strictly one #ifndef...#endif.  I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]

*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html