Change quoting to be compatible with future autoconf versions.

m4/c-bs-a.m4

@@ -1,4 +1,4 @@
-#serial 1
+#serial 2
 
 dnl From Paul Eggert.
 
@@ -6,16 +6,16 @@ AC_DEFUN(AC_C_BACKSLASH_A,
 [
   AC_CACHE_CHECK([whether backslash-a works in strings], ac_cv_c_backslash_a,
    [AC_TRY_COMPILE([],
-     [
-      changequote(, ) dnl
+     changequote(<<, >>)dnl
+     <<
 #if '\a' == 'a'
       syntax error;
 #endif
       char buf['\a' == 'a' ? -1 : 1];
       buf[0] = '\a';
       return buf[0] != "\a"[0];
-      changequote([, ])dnl
-     ],
+     >>,
+     changequote([, ])dnl
      ac_cv_c_backslash_a=yes,
      ac_cv_c_backslash_a=no)])
   if test $ac_cv_c_backslash_a = yes; then