autotools: Remove "auto" value of --with-ecmult-gen-kb option

"auto" implies that a value is being chosen based on build system
introspection or host system capabilities. However, for the
`--with-ecmult-gen-kb` option, the value "auto" is hardcoded, which
might lead to confusion.

This change replaces "auto" with a more appropriate default value.

.cirrus.yml

@@ -11,7 +11,7 @@ env:
   BUILD: check
   ### secp256k1 config
   ECMULTWINDOW: 15
-  ECMULTGENKB: auto
+  ECMULTGENKB: 22
   ASM: no
   WIDEMUL: auto
   WITH_VALGRIND: yes