Check if the compiler knows about the __malloc__ attribute.

m4/attribute.m4

@@ -0,0 +1,25 @@
+dnl Check to find out whether function attributes are supported.
+dnl If they are not, #define them to be nothing.
+
+AC_DEFUN(tinc_ATTRIBUTE,
+[
+  AC_CACHE_CHECK([for working $1 attribute], tinc_cv_attribute_$1,
+  [ 
+    tempcflags="$CFLAGS"
+    CFLAGS="$CFLAGS -Wall -Werror"
+    AC_COMPILE_IFELSE(
+      [AC_LANG_SOURCE(
+        [void test(void) __attribute__ (($1));
+	 void test(void) { return; }
+	],
+       )],
+       [tinc_cv_attribute_$1=yes],
+       [tinc_cv_attribute_$1=no]
+     )
+     CFLAGS="$tempcflags"
+   ])
+
+   if test ${tinc_cv_attribute_$1} = no; then
+     AC_DEFINE([$1], [], [Defined if the $1 attribute is not supported.])
+   fi
+])