Fix header_hygiene CI: skip pthread guards on libstdc++ in thread mode

CI on Linux failed `header_hygiene` because libstdc++ in -D_REENTRANT mode
(set unconditionally by configure.ac) routes std::thread support through
<bits/gthr-default.h>, which #includes <pthread.h> directly. That defines
_PTHREAD_H once any STL container header (<memory>, <string>, <vector>, ...)
is reached -- so the runtime sentinel reported a leak even though no
libhttpserver public header pulls <pthread.h> in.

This is the same STL implementation detail we already exempt for libc++
via _LIBCPP_VERSION; the original test comment claimed libstdc++ did not
have this property, which was incorrect for thread-enabled builds.

Extend the existing skip to also fire on libstdc++ in thread mode by
testing _GLIBCXX_HAS_GTHREADS, and update the matching comment in
Makefile.am so the asymmetry between the runtime sentinel and the
preprocessor-grep gate (which already excludes pthread) is documented
consistently. The guard still fires under STLs that don't route
std::thread through pthread (e.g. MSVC's Microsoft STL).

Verified locally:
  - macOS libc++: header_hygiene PASS (unchanged).
  - gcc:14 libstdc++ -D_REENTRANT: pre-fix the test reproduces the exact
    CI failure ("1 forbidden header(s) leaked"); post-fix it PASSes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: etr

Makefile.am

@@ -196,16 +196,23 @@ check-install-layout:
 # Cross-reference: keep HEADER_HYGIENE_FORBIDDEN in sync with the
 # #ifdef ladder in test/unit/header_hygiene_test.cpp.
 #
-# TASK-020 caveat (libc++): <pthread.h> is intentionally absent from
-# the forbidden list below. libc++ (Apple's default STL on macOS)
-# unconditionally includes <pthread.h> from any STL container header
-# (<vector>, <string>, <map>, etc.) via its
-# <__thread/support/pthread.h> backend. The resulting `# N "...pthread.h"`
-# line markers therefore appear in the preprocessed output even though
-# libhttpserver itself does not include <pthread.h>. The runtime
-# sentinel test/unit/header_hygiene_test.cpp keeps the pthread guard
-# but skips it when _LIBCPP_VERSION is defined, so the same invariant
-# is still enforced under libstdc++ and other STLs.
+# TASK-020 caveat (libc++ AND libstdc++ in thread mode): <pthread.h>
+# is intentionally absent from the forbidden list below. Both
+# mainstream STLs unconditionally pull <pthread.h> in from any STL
+# container header (<vector>, <string>, <map>, etc.) when threading
+# is enabled:
+#   - libc++ (Apple's default STL on macOS) routes through
+#     <__thread/support/pthread.h>.
+#   - libstdc++ in thread-enabled mode (which is the default whenever
+#     -D_REENTRANT is set, as configure.ac does) routes through
+#     <bits/gthr-default.h>, which #include <pthread.h> directly.
+# The resulting `# N "...pthread.h"` line markers therefore appear in
+# the preprocessed output even though libhttpserver itself does not
+# include <pthread.h>. The runtime sentinel
+# test/unit/header_hygiene_test.cpp keeps the pthread guards but skips
+# them on both libc++ (_LIBCPP_VERSION) and libstdc++ in thread mode
+# (_GLIBCXX_HAS_GTHREADS), so the guards still fire on STLs that don't
+# route std::thread through pthread (e.g. MSVC's Microsoft STL).
 # ---------------------------------------------------------------------------
 
 HEADER_HYGIENE_FORBIDDEN = microhttpd\.h|gnutls/gnutls\.h|sys/socket\.h|sys/uio\.h

test/unit/header_hygiene_test.cpp

@@ -59,16 +59,20 @@
 // red states during M2-M5 -- the leaks must be removed in production
 // code, not here.
 //
-// TASK-020 caveat (libc++/Apple): libc++ (Apple's default STL)
-// unconditionally includes <pthread.h> from any STL container header
-// (<vector>, <string>, <map>, etc.) via its
-// <__thread/support/pthread.h> backend. That defines _PTHREAD_H even
-// when libhttpserver itself does not include <pthread.h>. This is an
-// STL implementation detail, not a libhttpserver leak. We therefore
-// skip the pthread guards on libc++ (detected via _LIBCPP_VERSION).
-// On libstdc++ and other STLs, the same containers do NOT pull in
-// <pthread.h> transitively, so the guards remain a meaningful leak
-// signal there.
+// TASK-020 caveat (libc++ AND libstdc++ in thread mode): both
+// mainstream STLs pull <pthread.h> in transitively from any STL
+// container header (<vector>, <string>, <map>, etc.) when threading
+// is enabled, so _PTHREAD_H / _PTHREAD_H_ being defined is an STL
+// implementation detail, not a libhttpserver leak:
+//   - libc++ (Apple's default STL on macOS) routes through
+//     <__thread/support/pthread.h>.
+//   - libstdc++ in thread-enabled mode (which is the default whenever
+//     -D_REENTRANT is set, as configure.ac does) routes through
+//     <bits/gthr-default.h>, which #include <pthread.h> directly.
+// We therefore skip the pthread guards on both STLs (detected via
+// _LIBCPP_VERSION and _GLIBCXX_HAS_GTHREADS respectively). The guards
+// remain meaningful on STLs that don't use pthread for std::thread
+// (e.g. MSVC's Microsoft STL, which uses Win32 threading).
 //
 // Cross-reference: the same forbidden-header list is enforced via the
 // preprocessor-grep target `make check-hygiene` in the top-level
@@ -86,12 +90,14 @@ int main() {
     ++leaks;
 #endif
 
-// TASK-020: libc++ unconditionally drags <pthread.h> in from any STL
-// container header via its <__thread/support/pthread.h> backend, so
-// _PTHREAD_H / _PTHREAD_H_ defined on libc++ is not a libhttpserver
-// leak signal -- it is an STL implementation detail. Skip these guards
-// on libc++; keep them strict on libstdc++ and other STLs.
-#ifndef _LIBCPP_VERSION
+// TASK-020: both libc++ and libstdc++ (in thread-enabled mode, which
+// is the default whenever -D_REENTRANT is set) unconditionally drag
+// <pthread.h> in from any STL container header. _PTHREAD_H /
+// _PTHREAD_H_ being defined under either STL is an implementation
+// detail, not a libhttpserver leak. Skip these guards on both;
+// keep them strict on STLs that don't route std::thread through
+// pthread (e.g. MSVC's Microsoft STL).
+#if !defined(_LIBCPP_VERSION) && !defined(_GLIBCXX_HAS_GTHREADS)
 #ifdef _PTHREAD_H
     std::fprintf(stderr, "LEAK: <pthread.h> reached the consumer TU (glibc/musl guard _PTHREAD_H)\n");
     ++leaks;
@@ -101,7 +107,7 @@ int main() {
     std::fprintf(stderr, "LEAK: <pthread.h> reached the consumer TU (macOS/BSD guard _PTHREAD_H_)\n");
     ++leaks;
 #endif
-#endif  // !_LIBCPP_VERSION
+#endif  // !_LIBCPP_VERSION && !_GLIBCXX_HAS_GTHREADS
 
 #ifdef GNUTLS_GNUTLS_H
     std::fprintf(stderr, "LEAK: <gnutls/gnutls.h> reached the consumer TU (guard GNUTLS_GNUTLS_H)\n");