diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FD_MASK.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FD_MASK.h
new file mode 100644
index 0000000..81315e3
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FD_MASK.h
@@ -0,0 +1,16 @@
+// HAVE_FD_MASK : BUILD2_AUTOCONF_LIBC_VERSION
+
+#ifndef BUILD2_AUTOCONF_LIBC_VERSION
+#  error BUILD2_AUTOCONF_LIBC_VERSION appears to be conditionally included
+#endif
+
+#undef HAVE_FD_MASK
+
+/* Extremely little information, other than it being very old... */
+#if defined(__FreeBSD__)            || \
+    defined(__OpenBSD__)            || \
+    defined(__NetBSD__)             || \
+    defined(__GLIBC__)              || \
+    defined(BUILD2_AUTOCONF_MACOS)
+#  define HAVE_FD_MASK 1
+#endif