Perform build and magnum tuning

Building o//third_party/python now takes 5 seconds on my PC

This change works towards modifying Python to use runtime dispatching
when appropriate. For example, when loading the magnums in the socket
module, it's a good idea to check if the magnum is zero, because that
means the local system platform doesn't support it.

third_party/python/Objects/stringlib/count.inc

@@ -0,0 +1,28 @@
+/* clang-format off */
+/* stringlib: count implementation */
+
+#ifndef STRINGLIB_FASTSEARCH_H
+#error must include fastsearch.inc before including this module
+#endif
+
+Py_LOCAL_INLINE(Py_ssize_t)
+STRINGLIB(count)(const STRINGLIB_CHAR* str, Py_ssize_t str_len,
+                const STRINGLIB_CHAR* sub, Py_ssize_t sub_len,
+                Py_ssize_t maxcount)
+{
+    Py_ssize_t count;
+
+    if (str_len < 0)
+        return 0; /* start > len(str) */
+    if (sub_len == 0)
+        return (str_len < maxcount) ? str_len + 1 : maxcount;
+
+    count = FASTSEARCH(str, str_len, sub, sub_len, maxcount, FAST_COUNT);
+
+    if (count < 0)
+        return 0; /* no match */
+
+    return count;
+}
+
+