Fix ntlm feature broken by libcurl 8.20's NTLM opt-in flip

[ns-ylambert]

Summary

libcurl 8.20 (released 2026-04-29) flipped NTLM from opt-out to opt-in behind CURL_ENABLE_NTLM (curl/curl#20698, commit cc0c83c5f853). curl-sys picked up the libcurl bump in #649 (2026-05-01) but didn't propagate the opt-in contract: the ntlm cargo feature (introduced by #428, commit f0bbf37241e2) still only compiles the NTLM source files. From curl-sys 0.4.90+curl-8.20.0 onward, --features curl-sys/ntlm builds cleanly but exports zero NTLM symbols: the feature silently does nothing at runtime.

Fix

curl-sys/build.rs, inside the if cfg!(feature = "ntlm") { ... } block:

• Add .define("CURL_ENABLE_NTLM", None): the macro libcurl now requires to activate NTLM.
• Drop the else-branch defining CURL_DISABLE_NTLM: same curl PR that flipped the gate removed every reference to that macro from libcurl's source, docs, cmake, and configure logic; it's dead.

Cross-platform (not target-scoped): whoever activates curl-sys/ntlm opts in on any platform.

Also adds a regression test in src/lib.rs's mod tests: is_ntlm_support_activated asserts Version::feature_ntlm() == true when the crate's ntlm feature is enabled. Fails on any curl-sys 0.4.90+curl-8.20.0 or later without this fix.

Verification

• cargo check -p curl-sys --features static-curl,ntlm --target x86_64-pc-windows-msvc succeeds; the resulting binary now contains NTLM symbols.
• cargo test --features static-curl,ntlm --target x86_64-pc-windows-msvc is_ntlm_support_activated → test tests::is_ntlm_support_activated ... ok.
• Existing CI matrix passes unchanged.

Note on CI coverage of the regression test

ci/run.sh currently compiles the new test in the --no-run pass at line 36 (cargo test --no-run --features static-curl,protocol-ftp,ntlm) but does not execute it. No line in the if [ -z "$NO_RUN" ] block enables --features ntlm. Adding

cargo test --target $TARGET --features static-curl,ntlm $features

around line 41 would promote the test from a compile-verified reproducer to a CI-executed guard against future regressions of the same class. Kept out of this PR to keep the diff minimal; happy to add if desired.

Related

• Fixes the modern manifestation of NTLM authentication on Windows does not work #359 ("NTLM authentication on Windows does not work"). Original cause in 2023 was different — the reporter hadn't enabled --features ntlm at all — but the same runtime error ("A requested feature, protocol or option was not found built-in in this libcurl due to a build-time decision") now hits users who correctly enable the feature but are on curl-sys 0.4.90+curl-8.20.0 or later.
• Regression landed in Update curl to 8.20.0 #649 (2026-05-01, the curl 8.20 bump). The PR body flagged NTLM's uncertain future ("the next release will likely remove NTLM") but missed that 8.20 itself had already flipped the gate to opt-in.

[ns-ylambert]

Rebased onto current main (curl 8.22.0); CI green. This is the first of a four-part series (#665, #666, #667, #668) fixing windows-static-ssl and restoring SSPI on non-Schannel backends (each PR is kept independent from others).

Ready for review whenever you have a moment, happy to restructure if a different split is easier.