autotools: Remove "auto" value of --with-ecmult-window option

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

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

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ env:
   MAKEFLAGS: '-j4'
   BUILD: 'check'
   ### secp256k1 config
-  ECMULTWINDOW: 'auto'
+  ECMULTWINDOW: 15
   ECMULTGENKB: 'auto'
   ASM: 'no'
   WIDEMUL: 'auto'