$NetBSD: patch-configure.ac,v 1.2 2021/08/21 19:22:12 mcf Exp $

--- configure.ac.orig	2015-01-02 18:43:42.000000000 +0000
+++ configure.ac
@@ -52,6 +52,9 @@ dnl
 AC_ARG_ENABLE(futex,	AS_HELP_STRING([--enable-futex], [Enable futexes (default: auto)]),
 		[FUTEX=$enableval], [FUTEX=auto])
 
+AC_ARG_ENABLE(semaphore,	AS_HELP_STRING([--enable-semaphore], [Enable POSIX named semaphores (default: no)]),
+		[SEMAPHORE=$enableval], [SEMAPHORE=no])
+
 if test "x$FUTEX" = "xauto"; then
 	AC_CHECK_HEADER([linux/futex.h], [FUTEX=yes])
 fi
@@ -65,23 +68,36 @@ if test "x$FUTEX" = "xauto"; then
 	fi
 fi
 
+if test "x$SEMAPHORE" = "xyes"; then
+	AC_CHECK_HEADER([semaphore.h], [], [AC_MSG_ERROR([No platform support for POSIX named semaphores])])
+fi
+
 if test "x$FUTEX" = "xyes"; then
 	PTHREAD=no
 	AC_DEFINE(HAVE_FUTEX,1,[Use futexes])
 else
-	PTHREAD=yes
-	AC_DEFINE(HAVE_PTHREAD,1,[Use pthread primitives])
+	if test "x$SEMAPHORE" = "xyes"; then
+		PTHREAD=no
+		AC_DEFINE(HAVE_SEMAPHORE,1,[Use semaphores])
+	else
+		PTHREAD=yes
+		AC_DEFINE(HAVE_PTHREAD,1,[Use pthread primitives])
+	fi
 fi
 
 PTHREAD_LIBS=
 if test "x$PTHREAD" = "xyes"; then
 	AC_CHECK_LIB(pthread,pthread_create,[PTHREAD_LIBS=-lpthread],[PTHREAD_LIBS=])
 fi
+if test "x$SEMAPHORE" = "xyes"; then
+	AC_CHECK_LIB(rt,sem_open,[PTHREAD_LIBS=-lrt])
+fi
 
 AC_SUBST([PTHREAD_LIBS])
 
 AM_CONDITIONAL([FUTEX], [test x"$FUTEX" = xyes])
 AM_CONDITIONAL([PTHREAD], [test x"$PTHREAD" = xyes])
+AM_CONDITIONAL([SEMAPHORE], [test x"$SEMAPHORE" = xyes])
 
 PKG_CHECK_MODULES(XPROTO, xproto)