By exposing and reusing existing custom type via `vtls/openss.h`.
Also:
- GHA/curl-for-win: test in CI by enabling building tests.
Cost is 45s per job, so limit it to the gcc job.
Seen with Windows x64 gcc (not tested in CI prior to this patch):
```
tests/libtest/lib1587.c:50:7: error: conversion from 'opt1587' {aka 'long long unsigned int'} to 'long int' may change value [-Werror=conversion]
50 | SSL_CTX_set_options(info->internals, opts);
| ^~~~~~~~~~~~~~~~~~~
tests/libtest/lib1587.c:59:7: error: conversion from 'opt1587' {aka 'long long unsigned int'} to 'long int' may change value [-Werror=conversion]
59 | SSL_set_options(info->internals, opts);
| ^~~~~~~~~~~~~~~
```
Ref: https://github.com/curl/curl/actions/runs/28258372229/job/83727170184?pr=22195#step:3:4884
Ref: da2f05e6f6#22198
Ref: 6163566461#22197
Follow-up to 3e40ccb875#21290
Follow-up to 2db8ae480f#17809#17801Closes#22195
This should return a SSL_CTX pointer but it was accidentally broken.
Verify with test 1587
Follow-up to 2db8ae480f
Spotted by Codex Security
Closes#21290
