ircd::fs: Add autoconfs for sync(2) etc.

configure.ac

@@ -1382,6 +1382,8 @@ AC_CHECK_FUNCS([          \
 	pwritev2              \
 	strlcat               \
 	strlcpy               \
+	sync                  \
+	syncfs                \
 ])
 
 AC_SEARCH_LIBS(dlinfo, dl, AC_DEFINE(HAVE_DLINFO, 1, [Define if you have dlinfo]))

ircd/fs.cc

@@ -700,10 +700,12 @@ ircd::fs::sync(const fd &fd,
 		"fs::sync(fd:%d)", int(fd)
 	};
 
-	#ifdef __linux__
+	#if defined(HAVE_SYNCFS)
 		syscall(::syncfs, fd);
-	#else
+	#elif defined(HAVE_SYNC)
 		syscall(::sync);
+	#else
+		#error "Missing sync(2) on this platform."
 	#endif
 }