Do not use syscall(2) on OS X 10.12 (deprecated).

configure.ac

@@ -1350,6 +1350,23 @@ if test "x${je_cv_mach_absolute_time}" = "xyes" ; then
   AC_DEFINE([JEMALLOC_HAVE_MACH_ABSOLUTE_TIME])
 fi
 
+dnl Check if syscall(2) is usable.  Treat warnings as errors, so that e.g. OS X
+dnl 10.12's deprecation warning prevents use.
+SAVED_CFLAGS="${CFLAGS}"
+JE_CFLAGS_APPEND([-Werror])
+JE_COMPILABLE([syscall(2)], [
+#define _GNU_SOURCE
+#include <sys/syscall.h>
+#include <unistd.h>
+], [
+	syscall(SYS_write, 2, "hello", 5);
+],
+              [je_cv_syscall])
+CFLAGS="${SAVED_CFLAGS}"
+if test "x$je_cv_syscall" = "xyes" ; then
+  AC_DEFINE([JEMALLOC_HAVE_SYSCALL], [ ])
+fi
+
 dnl Check if the GNU-specific secure_getenv function exists.
 AC_CHECK_FUNC([secure_getenv],
               [have_secure_getenv="1"],

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -71,6 +71,9 @@
  */
 #undef JEMALLOC_OSSPIN
 
+/* Defined if syscall(2) is available. */
+#undef JEMALLOC_HAVE_SYSCALL
+
 /*
  * Defined if secure_getenv(3) is available.
  */

src/pages.c

@@ -219,7 +219,7 @@ os_overcommits_proc(void)
 	char buf[1];
 	ssize_t nread;
 
-#ifdef SYS_open
+#if defined(JEMALLOC_HAVE_SYSCALL) && defined(SYS_open)
 	fd = (int)syscall(SYS_open, "/proc/sys/vm/overcommit_memory", O_RDONLY);
 #else
 	fd = open("/proc/sys/vm/overcommit_memory", O_RDONLY);
@@ -227,13 +227,13 @@ os_overcommits_proc(void)
 	if (fd == -1)
 		return (false); /* Error. */
 
-#ifdef SYS_read
+#if defined(JEMALLOC_HAVE_SYSCALL) && defined(SYS_read)
 	nread = (ssize_t)syscall(SYS_read, fd, &buf, sizeof(buf));
 #else
 	nread = read(fd, &buf, sizeof(buf));
 #endif
 
-#ifdef SYS_close
+#if defined(JEMALLOC_HAVE_SYSCALL) && defined(SYS_close)
 	syscall(SYS_close, fd);
 #else
 	close(fd);

src/util.c

@@ -49,7 +49,7 @@ static void
 wrtmessage(void *cbopaque, const char *s)
 {
 
-#ifdef SYS_write
+#if defined(JEMALLOC_HAVE_SYSCALL) && defined(SYS_write)
 	/*
 	 * Use syscall(2) rather than write(2) when possible in order to avoid
 	 * the possibility of memory allocation within libc.  This is necessary