m4: drop redundant conditions in TLS library detections

[vszakats]

Omit checking OPT_<BACKEND> against no twice.

Also:

• openssl: drop stray OPT_OPENSSL=off check.
  Follow-up to 68d89f2 configure: make the TLS library choice(s) explicit #6897
• rustls: drop no-op line.
  Follow-up to 9c42098 m4: fix rustls pkg-config codepath #13202
• gnutls: fix casing in comment.
• merge if branches where possible after these changes.

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https://github.com/curl/curl/pull/21781/files?w=1

[cpu]

I'm not enough of an autotools knower to evaluate on its face that this trailing RUSTLS_ENABLED being removed is a no-op. Can you help me understand why it is?

It looks like it was added fairly recently in 9c42098 as part of a bug fix and it'd be a shame to regress accidentally.

[vszakats]

There are two signals that make me think this was a placebo/mistake:

• configure generates identical whether this line is there or not.
• no other, functionally equivalent m4 module have this.

+1 this line by itself just evaluates to a value, which value is not consumed
by anything. Maybe it was meant as a return value of the m4 file, but AFAIU
this is not how m4 files work.

[cpu]

These are convincing observations, thank you.

[Copilot]

Pull request overview

This PR simplifies curl’s Autoconf TLS backend detection macros by removing redundant OPT_<BACKEND> != no conditions, cleaning up a few backend-specific no-op/stray checks, and slightly restructuring conditionals for readability while keeping behavior equivalent.

Changes:

• Remove duplicated OPT_<BACKEND> != no checks inside TLS backend detection blocks.
• OpenSSL: drop the special-case OPT_OPENSSL=off check and consolidate error handling paths.
• Minor cleanup: remove a Rustls no-op line, fix GnuTLS comment casing, and trim trailing whitespace in AmiSSL macro.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
m4/curl-wolfssl.m4	Removes redundant nesting and keeps wolfSSL detection/error flow intact.
m4/curl-schannel.m4	Drops redundant OPT_SCHANNEL check inside an already-guarded block.
m4/curl-rustls.m4	Simplifies the final failure check and removes a no-op trailing token.
m4/curl-openssl.m4	Removes OPT_OPENSSL=off special-case and merges conditional branches around failure handling.
m4/curl-mbedtls.m4	Removes redundant outer condition and tightens the structure of the detection flow.
m4/curl-gnutls.m4	Removes redundant nesting and normalizes comment casing while preserving detection logic.
m4/curl-amissl.m4	Removes a trailing whitespace-only line.

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