Merge bitcoin/bitcoin#31008: depends: Print ready-to-use --toolchain option for CMake invocation

605926d depends: Print ready-to-use `--toolchain` option for CMake invocation (Hennadii Stepanov)

Pull request description:

  Requested in bitcoin/bitcoin#30997 (comment):
  > P.S. it would be nice if `make` in `depends` provides the incantation needed for the configure stage.

  An example of a build log with this PR:
  ```
  $ make --no-print-directory -C depends -j16 NO_QT=1 NO_WALLET=1 NO_UPNP=1 NO_NATPMP=1 NO_ZMQ=1 NO_USDT=1 LOG=1
  Extracting boost...
  /home/hebasto/git/bitcoin/depends/sources/boost_1_81_0.tar.gz: OK
  Preprocessing boost...
  Configuring boost...
  Building boost...
  Staging boost...
  Postprocessing boost...
  Caching boost...
  Extracting libevent...
  /home/hebasto/git/bitcoin/depends/sources/libevent-2.1.12-stable.tar.gz: OK
  Preprocessing libevent...
  Configuring libevent...
  Building libevent...
  Staging libevent...
  Postprocessing libevent...
  Caching libevent...
  copying packages: boost libevent
  to: /home/hebasto/git/bitcoin/depends/x86_64-pc-linux-gnu
  To build Bitcoin Core with these packages, pass '--toolchain /home/hebasto/git/bitcoin/depends/x86_64-pc-linux-gnu/toolchain.cmake' to the first CMake invocation.
  ```

ACKs for top commit:
  Sjors:
    tACK 605926d
  theuni:
    ACK 605926d
  jarolrod:
    ACK 605926d
  pablomartin4btc:
    ACK 605926d

Tree-SHA512: 658358cb12a2409486962273d418f8cfa9609c84c6e70b426250173925c0938a6eba0dca72f43d78cc63c9cb927514c959473da04bdcda714228c5d8c248c1fe

Author: fanquake

depends/Makefile

@@ -189,6 +189,7 @@ $(host_prefix)/.stamp_$(final_build_id): $(native_packages) $(packages)
 	echo copying packages: $^
 	echo to: $(@D)
 	cd $(@D); $(foreach package,$^, $(build_TAR) xf $($(package)_cached); )
+	echo To build Bitcoin Core with these packages, pass \'--toolchain $(@D)/toolchain.cmake\' to the first CMake invocation.
 	touch $@
 
 ifeq ($(host),$(build))