build: auto-detect the presence of the openat2() syscall

Let configure detect if the openat2() syscall is supported by the kernel
headers we are building against. Do not attempt to use openat2() if
support is not present.

Users can still disable using the openat2() syscall manually if so
desired.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>

configure.ac

@@ -103,10 +103,6 @@ dnl (and coverage-counted) without needing a pre-5.6 kernel. Behaviour-neutral
 dnl by default (the knob only REMOVES a tier when explicitly disabled).
 AC_ARG_ENABLE(openat2,
 	AS_HELP_STRING([--disable-openat2],[do not use Linux openat2(RESOLVE_BENEATH); force the portable resolver (for exercising the fallback tier)]))
-if test x"$enable_openat2" != x"no"; then
-	AC_DEFINE([HAVE_OPENAT2], 1,
-		[Define to use Linux openat2(RESOLVE_BENEATH) in secure_relative_open where available.])
-fi
 
 AC_MSG_CHECKING([if md2man can create manpages])
 if test x"$ac_cv_path_PYTHON3" = x; then
@@ -357,6 +353,19 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[ ]], [[return 0;]])],
 CFLAGS="$OLD_CFLAGS"
 AC_SUBST(NOEXECSTACK)
 
+AC_CACHE_CHECK([for openat2],rsync_cv_HAVE_OPENAT2,[
+    AC_COMPILE_IFELSE([
+        AC_LANG_PROGRAM([[#include <sys/syscall.h>]], [[int i = SYS_openat2]])
+    ],
+    [rsync_cv_HAVE_OPENAT2=yes], [rsync_cv_HAVE_OPENAT2=no])
+])
+if test x"$enable_openat2" != x"no"; then
+    if test x"$rsync_cv_HAVE_OPENAT2" = x"yes"; then
+      AC_DEFINE([HAVE_OPENAT2], 1,
+        [Define to use Linux openat2(RESOLVE_BENEATH) in secure_relative_open where available.])
+    fi
+fi
+
 # arrgh. libc in some old debian version screwed up the largefile
 # stuff, getting byte range locking wrong
 AC_CACHE_CHECK([for broken largefile support],rsync_cv_HAVE_BROKEN_LARGEFILE,[