-#if defined(__GNUC__) && !defined(C_ALLOCA)
-# ifndef alloca
-# define alloca __builtin_alloca
-# endif
-#else /* ! defined (__GNUC__) */
-# ifdef _AIX
- #pragma alloca
+/* This is a merge of code recommended in the autoconf-2.61 documentation
+ with that recommended in the autoconf-2.13 documentation, with added
+ tweaks to heed C_ALLOCA. */
+
+#if defined HAVE_ALLOCA_H && !defined C_ALLOCA
+# include <alloca.h>
+#else
+# if defined __GNUC__ && !defined C_ALLOCA
+# if !defined alloca
+# define alloca __builtin_alloca
+# endif
+# else
+# if defined _AIX
+/* Indented so that pre-ansi C compilers will ignore it, rather than
+ choke on it. Some versions of AIX require this to be the first
+ thing in the file except for comments and preprocessor directives. */
+ #pragma alloca