Dedupe and migrate MSVC-specific warning suppressions to `curl_setup.h`.
Make cmake set `_CRT_SECURE_NO_DEPRECATE` for examples and standalone
tests, and stop setting `_CRT_NONSTDC_NO_DEPRECATE` for them.
Details:
- drop version guards. On ancient MSVC version these macro are a no-op.
- move to `curl_setup.h` from `config-win32*.h`.
- sync macro values with CMake.
- cmake: stop setting them globally in favour of `curl_setup.h`.
- cmake: re-add these macros to `docs/examples` and `tests/http/clients`,
which do not use `curl_setup.h`.
- cmake: drop `_CRT_NONSTDC_NO_DEPRECATE` for examples and tests.
They build fine without.
- update comments.
Closes#15960
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If you would rather see the rendered version of the documentation, check out the
curl website's documentation section for
general curl stuff or the libcurl section for
libcurl related documentation.