autoreconf fails and it's a trivial fix to the generated configure, so let's
just patch that instead. This is fixed in autoconf itself in newer versions.

https://bugs.gentoo.org/879689
--- a/configure
+++ b/configure
@@ -6289,6 +6289,7 @@ cat confdefs.h >>conftest.$ac_ext
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
 #include <ctype.h>
+#include <stdlib.h>
 #if ((' ' & 0x0FF) == 0x020)
 # define ISLOWER(c) ('a' <= (c) && (c) <= 'z')
 # define TOUPPER(c) (ISLOWER(c) ? 'A' + ((c) - 'a') : (c))