(partial) 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

depends/README.md

@@ -12,15 +12,18 @@ For example:
 
     make HOST=x86_64-w64-mingw32 -j4
 
-**Dash Core's configure script by default will ignore the depends output.** In
+**Dash Core's `configure` script by default will ignore the depends output.** In
 order for it to pick up libraries, tools, and settings from the depends build,
-you must point it at the appropriate `--prefix` directory generated by the
-build. In the above example, a prefix dir named x86_64-w64-mingw32 will be
-created. To use it for Dash:
+you must set the `CONFIG_SITE` environment variable to point to a `config.site` settings file.
+In the above example, a file named `depends/x86_64-w64-mingw32/share/config.site` will be
+created. To use it during compilation:
 
-    ./configure --prefix=$PWD/depends/x86_64-w64-mingw32
+    CONFIG_SITE=$PWD/depends/x86_64-w64-mingw32/share/config.site ./configure
 
-Common `host-platform-triplets` for cross compilation are:
+The default install prefix when using `config.site` is `--prefix=depends/<host-platform-triplet>`,
+so depends build outputs will be installed in that location.
+
+Common `host-platform-triplet`s for cross compilation are:
 
 - `i686-pc-linux-gnu` for Linux 32 bit
 - `x86_64-pc-linux-gnu` for x86 Linux
@@ -138,4 +141,3 @@ This is an example command for a default build with no disabled dependencies:
 
 - [description.md](description.md): General description of the depends system
 - [packages.md](packages.md): Steps for adding packages
-

doc/build-unix.md

@@ -331,7 +331,8 @@ To build executables for ARM:
     cd depends
     make HOST=arm-linux-gnueabihf NO_QT=1
     cd ..
-    ./configure --prefix=$PWD/depends/arm-linux-gnueabihf --enable-reduce-exports LDFLAGS=-static-libstdc++
+    ./autogen.sh
+    CONFIG_SITE=$PWD/depends/arm-linux-gnueabihf/share/config.site ./configure --enable-reduce-exports LDFLAGS=-static-libstdc++
     make