tests: check decoding of accept4 syscall

* configure.ac (AC_CHECK_FUNCS): Add accept4.
* tests/accept4.c: New file.
* tests/accept4.test: New test.
* tests/.gitignore: Add accept4.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add accept4.test.
diff --git a/configure.ac b/configure.ac
index 97b75be..f70a7bd 100644
--- a/configure.ac
+++ b/configure.ac
@@ -282,6 +282,7 @@
 AC_HEADER_STDC
 
 AC_CHECK_FUNCS(m4_normalize([
+	accept4
 	fanotify_mark
 	fopen64
 	fork