Fix GH-20920: fix build with LibreSSL

[gvozdetsky]

Fixes #20920.

Supersedes #20932 — thanks to @jordikroon for the original patch and for
letting me pick this up.

This addresses two of the three LibreSSL build failures reported in GH-20920,
one commit each.

1. ENGINE_cleanup() called without a prototype

The call used to be guarded by #ifndef OPENSSL_NO_ENGINE. The v1/v3 backend
split in d662ab5 moved it from ext/openssl/openssl.c into
ext/openssl/openssl_backend_v1.c and kept only the surrounding
#ifdef LIBRESSL_VERSION_NUMBER. LibreSSL defines OPENSSL_NO_ENGINE, so
<openssl/engine.h> is not included and the call is compiled without a
prototype — a warning with older compilers, an error with GCC 14 and later,
which default to C23. That is what @bkuhls hit with gcc15.

This restores the guard, so the call matches the condition of its include,
as suggested by @botovq.

2. PKCS7_NO_DUAL_CONTENT is not always available

The constant was only added in LibreSSL 4.3.0. Following @sthen's suggestion,
it is guarded on the macro itself rather than on LIBRESSL_VERSION_NUMBER, so
that LibreSSL 4.3.0 and later — which added the macro in response to this very
report (openbsd/src@a7d6713) — keep exposing it.

Testing

Built with --enable-debug --enable-zts against LibreSSL 4.2.1 and 4.3.2
(both built from source) and against OpenSSL 3.0.13:

• LibreSSL 4.2.1 reproduces both errors on PHP-8.5 and compiles cleanly with
  this PR.
• Under LibreSSL 4.3.2, the set of failing ext/openssl tests is identical
  with and without this PR (26 pre-existing LibreSSL incompatibilities:
  removed TLS 1.0/1.1, DH/DSA differences, openssl_error_string() formats,
  security_level). So this PR neither fixes nor breaks any of them.
• Under LibreSSL 4.3.2 the constant is still registered
  (PKCS7_NO_DUAL_CONTENT === 65536), confirming the macro-based guard.
• On OpenSSL 3.0.13: ext/openssl 214 passed, 0 failed, 15 skipped — no
  behaviour change, since OPENSSL_NO_ENGINE is not defined there and the
  constant exists.

This is not sufficient on its own

Linking still fails under LibreSSL with:

undefined reference to `sk_X509_new_reserve'
  ext/openssl/openssl_backend_common.c:714 (php_openssl_load_all_certs_from_file)

#21430 fixes that, and it is needed on PHP-8.5 as well, not only on master.
With #21430 applied on top of this PR, the build against LibreSSL 4.2.1
completes successfully.

@bkuhls, @bneumeier — could you confirm this matches what you see on your
LibreSSL setups?