fftw2 is unsupported upstream: last release 1999, maintenance has switch to version fftw 3. Therefore, we need to apply a custom patch to avoid C89isms.

diff --git a/configure b/configure
index f5bab28675937533..5af49397a60b4811 100755
--- a/configure
+++ b/configure
@@ -4459,8 +4459,8 @@ main ()
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
         || toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext

This check seems to have been built into earlier autoconf versions, so the patch will likely be required elsewhere, too.