build: make sure we can overwrite config.{guess,sub}

Since ea7b852 (bitcoin#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} subsequently fails.

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

fixes bitcoin#27873: Can't copy to 'build-aux/config.guess' in autoconf.sh: Permission denied

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