Fix building against glibc older than 2.29, which has no
posix_spawn_file_actions_addchdir_np (the symbol is genuinely absent from
libc.so, so no feature-test macro helps). Affects manylinux2014 (glibc 2.17)
and manylinux_2_28, and was reported on RHEL 8.1. A requested process_cwd now
fails with ENOSYS there instead of failing the build.

Upstream PR: https://github.com/sheredom/subprocess.h/pull/104
Applied locally by scripts/sync_vendor.py until it is merged upstream.

(the README.md and test/ changes from the PR are omitted, we only vendor
subprocess.h; rebased on top of patch-bsd.patch, so apply it after that one)

diff --git a/subprocess.h b/subprocess.h
index 1ef424a..c363393 100644
--- a/subprocess.h
+++ b/subprocess.h
@@ -274,6 +274,21 @@ subprocess_weak int subprocess_alive(struct subprocess_s *const process);
 #include <unistd.h>
 #endif
 
+/* Whether subprocess_create_ex can honour process_cwd. glibc only gained
+   posix_spawn_file_actions_addchdir_np in 2.29. Define this yourself to
+   override the detection, for instance on musl older than 1.1.24. */
+#if !defined(SUBPROCESS_HAVE_CWD)
+#if defined(__GLIBC__)
+#if __GLIBC_PREREQ(2, 29)
+#define SUBPROCESS_HAVE_CWD 1
+#else
+#define SUBPROCESS_HAVE_CWD 0
+#endif
+#else
+#define SUBPROCESS_HAVE_CWD 1
+#endif
+#endif
+
 #if defined(_WIN32)
 
 #include <wchar.h>
@@ -1219,6 +1234,8 @@ cleanup:
   if (process_cwd) {
 #if defined(__NetBSD__) || (defined(__APPLE__) && MAC_OS_X_VERSION_MIN_REQUIRED >= 260000)
     posix_error = posix_spawn_file_actions_addchdir(&actions, process_cwd);
+#elif !SUBPROCESS_HAVE_CWD
+    posix_error = ENOSYS;
 #else
 #if defined(__APPLE__) && defined(__clang__)
 #pragma clang diagnostic push