Merge bitcoin/bitcoin#27875: build: make sure we can overwrite config.{guess,sub} before doing so

fc6c17b build: make sure we can overwrite config.{guess,sub} (0xb10c)

Pull request description:

  Since ea7b852 (#26422), `autogen.sh` overwrites the `build-aux/config.{guess, sub}` files (installed there by `autoreconf`) with the `depends/config.{guess, sub}` files if these are newer.

  The `autoreconf` tool copies them from it's `share/autoconf/build-aux/` directory. Specifically on NixOS, the `share/autoconf/build-aux/` files are located in the nix-store and are read-only. `autoreconf` preserves the read-only permissions when copying. Overwriting them with our `depends/config.{guess, sub}` files subsequently fails.

  To make sure we can overwrite the files, set write permissions to the current user and group before overwriting. This fixes the problem on NixOS.

  fixes #27873

ACKs for top commit:
  dergoegge:
    tACK fc6c17b
  fanquake:
    ACK fc6c17b

Tree-SHA512: e8a31f739d5b598b2fe9fe6fc3d02303c117a6adccc49b8d0fea4980027a64f915a0e1e00e4788dce6113ef1b9ec9acf9e4164486f6e4904bad405f20b6746a0

Author: fanquake

autogen.sh

@@ -16,10 +16,14 @@ command -v autoreconf >/dev/null || \
 autoreconf --install --force --warnings=all
 
 if expr "'$(build-aux/config.guess --timestamp)" \< "'$(depends/config.guess --timestamp)" > /dev/null; then
+  chmod ug+w build-aux/config.guess
+  chmod ug+w src/secp256k1/build-aux/config.guess
   cp depends/config.guess build-aux
   cp depends/config.guess src/secp256k1/build-aux
 fi
 if expr "'$(build-aux/config.sub --timestamp)" \< "'$(depends/config.sub --timestamp)" > /dev/null; then
+  chmod ug+w build-aux/config.sub
+  chmod ug+w src/secp256k1/build-aux/config.sub
   cp depends/config.sub build-aux
   cp depends/config.sub src/secp256k1/build-aux
 fi