[SECP256K1] Fix insecure links

Summary:
Backport of [[bitcoin-core/secp256k1#858 | secp256k1#858]]

Update our specific files as well.

Test Plan:
Check it returns nothing:
  git grep http://

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9379

Author: rex4539

src/secp256k1/build-aux/m4/ax_prog_cc_for_build.m4

@@ -1,5 +1,5 @@
 # ===========================================================================
-#   http://www.gnu.org/software/autoconf-archive/ax_prog_cc_for_build.html
+#   https://www.gnu.org/software/autoconf-archive/ax_prog_cc_for_build.html
 # ===========================================================================
 #
 # SYNOPSIS

src/secp256k1/contrib/lax_der_parsing.c

@@ -1,8 +1,8 @@
-/**********************************************************************
- * Copyright (c) 2015 Pieter Wuille                                   *
- * Distributed under the MIT software license, see the accompanying   *
- * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
- **********************************************************************/
+/***********************************************************************
+ * Copyright (c) 2015 Pieter Wuille                                    *
+ * Distributed under the MIT software license, see the accompanying    *
+ * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
+ ***********************************************************************/
 
 #include <string.h>
 #include <secp256k1.h>