Merge bitcoin#17934: doc: Use CONFIG_SITE variable instead of --prefix option

223b1ba doc: Use CONFIG_SITE instead of --prefix (Hennadii Stepanov)

Pull request description:

  The current examples of `--prefix=...` option usage to point `configure` script to appropriate `depends` directory is not [standard](https://www.gnu.org/prep/standards/html_node/Directory-Variables.html). This causes some [confusion](bitcoin#16691) and a bit of inconvenience.

  Consider a CentOS 7 32 bit system. Packages `libdb4-devel`, `libdb4-cxx-devel`, `miniupnpc-devel` and `zeromq-devel` are unavailable from repos. After recommended build with depends:
  ```
  cd depends
  make
  cd ..
  ./autogen.sh
  ./configure --prefix=$PWD/depends/i686-pc-linux-gnu
  make
  ```
  a user is unable to `make install` compiled binaries neither locally (to `~/.local`) nor system-wide (to `/usr/local`) as `--prefix` is set already.

  Meanwhile, the standard approach with using [`config.site`](https://www.gnu.org/software/automake/manual/html_node/config_002esite.html) files allows both possibilities:

  ```
  cd depends
  make
  cd ..
  ./autogen.sh
  CONFIG_SITE=$PWD/depends/i686-pc-linux-gnu/share/config.site ./configure --prefix ~/.local
  make
  make install
  ```

  or

  ```
  CONFIG_SITE=$PWD/depends/i686-pc-linux-gnu/share/config.site ./configure
  make
  sudo make install  # install to /usr/local
  ```

  Moreover, this approach is used in [Gitian descriptors](https://github.com/bitcoin/bitcoin/tree/master/contrib/gitian-descriptors) already.

ACKs for top commit:
  practicalswift:
    ACK 223b1ba: patch looks correct
  fanquake:
    ACK 223b1ba

Tree-SHA512: 46d97924f0fc7e95ee4566737cf7c2ae805ca500e5c49af9aa99ecc3acede4b00329bc727a110aa1b62618dfbf5d1ca2234e736f16fbdf96d6ece5f821712f54

Author: fanquake

doc/multiprocess.md

@@ -24,7 +24,7 @@ The multiprocess feature requires [Cap'n Proto](https://capnproto.org/) and [lib
 ```
 cd <DASH_SOURCE_DIRECTORY>
 make -C depends NO_QT=1 MULTIPROCESS=1
-./configure --prefix=$PWD/depends/x86_64-pc-linux-gnu
+CONFIG_SITE=$PWD/depends/x86_64-pc-linux-gnu/share/config.site ./configure
 make
 src/dash-node -regtest -printtoconsole -debug=ipc
 DASHD=dash-node test/functional/test_runner.py