tidy-up: `curl_setup.h`, `curl_setup_once.h`, `config-win32ce.h`

tidy-up: `curl_setup.h`, `curl_setup_once.h`, `config-win32ce.h`

- drop unused `LIBIDN_REQUIRED_VERSION` macro.
  Unused since 9c91ec778104ae3b744b39444d544e82d5ee9ece

- drop compatibility error for `CURL_WANTS_CA_BUNDLE_ENV`.
  This macro was once set by `Makefile.mk` and Watcom makefiles.
  They are no longer supported, making the compatibility message moot.

  Follow-up to 7d7346519d8bff513f9ed9f0c5e709782b159e5a #1542 (2017)
  Follow-up to c2aeb1b3ba09af8b2e555eafa94dd9c534426bfb #1538 (2017)

- document last MSVC supporting the `!_MT` condition
  Ref: https://stackoverflow.com/questions/2278919/are-the-time-functions-of-msvc-thread-safe

- flatten an `#if` tree, prefer `#ifdef`.

- fix/adjust indentation, whitespace.

Closes #15967