[DOC] Update the depends README with dependencies and cmake instructions

Summary:
This diff updates the cross compilation instructions to use cmake and
ninja.
It also make the doc in sync with core with up to date dependencies.

Backport of core [[bitcoin/bitcoin#14264 | PR14264]] and [[bitcoin/bitcoin#16051 | PR16051]].

Test Plan: Read and test the instructions.

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: jasonbcox, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5394

depends/README.md

@@ -6,29 +6,79 @@ To build dependencies for the current arch+OS:
 
 To build for another arch/OS:
 
-    make HOST=host-platform-triplet
+    make build-<platform>
 
-For example:
+Where `<platform>` is one of the following:
+ - linux64
+ - linux32
+ - linux-arm
+ - linux-aarch64
+ - osx
+ - win32
+ - win64
 
-    make HOST=x86_64-w64-mingw32
+For example, building the dependencies for macOS:
 
-A prefix will be generated that's suitable for plugging into Bitcoin's
-configure. In the above example, a dir named x86_64-w64-mingw32 will be
-created. To use it for bitcoin-abc:
+    make build-osx
 
-    ./configure --prefix=`pwd`/depends/x86_64-w64-mingw32
+Note that it will use all the CPU cores available on the machine by default.
+This behavior can be changed by setting the `JOBS` environment variable (see
+below).
 
-Common `host-platform-triplets` for cross compilation are:
+To use the dependencies for building Bitcoin ABC, you need to set the platform
+file to be used by `cmake`.
+The platform files are located under `cmake/platforms/`.
+For example, cross-building for macOS (run from the project root):
 
-- `i686-w64-mingw32` for Win32
-- `x86_64-w64-mingw32` for Win64
-- `x86_64-apple-darwin14` for macOS
-- `arm-linux-gnueabihf` for Linux ARM 32 bit
-- `aarch64-linux-gnu` for Linux ARM 64 bit
+    mkdir build_osx
+    cd build_osx
+    cmake -GNinja .. -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/OSX.cmake
+    ninja
 
 No other options are needed, the paths are automatically configured.
 
-Dependency Options:
+### Install the required dependencies: Ubuntu & Debian
+
+#### Common to all arch/OS
+
+    sudo apt-get install build-essential autoconf automake cmake curl git libtool ninja-build patch pkg-config python3
+
+#### For macOS cross compilation
+
+    sudo apt-get install imagemagick libbz2-dev libcap-dev librsvg2-bin libtiff-tools python3-setuptools
+
+#### For Win32/Win64 cross compilation
+
+- see [build-windows.md](../doc/build-windows.md#cross-compilation-for-ubuntu-and-windows-subsystem-for-linux)
+
+#### For linux cross compilation
+
+Common linux dependencies:
+
+    sudo apt-get install gperf
+
+For linux 32 bits cross compilation:
+
+First add the i386 architecture to `dpkg`:
+
+    sudo dpkg --add-architecture i386
+    sudo apt-get update
+
+Then install the dependencies:
+
+    sudo apt-get install lib32stdc++-8-dev libc6-dev:i386
+
+For linux ARM cross compilation:
+
+    sudo apt-get install g++-arm-linux-gnueabihf
+
+For linux AARCH64 cross compilation:
+
+    sudo apt-get install g++-aarch64-linux-gnu
+
+
+### Dependency Options
+
 The following can be set when running make: make FOO=bar
 
     SOURCES_PATH: downloaded sources will be placed here
@@ -44,15 +94,19 @@ The following can be set when running make: make FOO=bar
     BUILD_ID_SALT: Optional salt to use when generating build package ids
     JOBS: Number of jobs to use for each package build
 
-If some packages are not built, for example `make NO_WALLET=1`, the appropriate
-options will be passed to bitcoin's configure. In this case, `--disable-wallet`.
+If some packages are not built, for example by building the depends with
+`make NO_WALLET=1`, the appropriate options should be set when building Bitcoin
+ABC using these dependencies.
+In this example, `-DBUILD_BITCOIN_WALLET=OFF` should be passed to the `cmake`
+command line to ensure that the build will not fail due to missing dependencies.
 
 Additional targets:
 
     download: run 'make download' to fetch all sources without building them
     download-osx: run 'make download-osx' to fetch all sources needed for macOS builds
     download-win: run 'make download-win' to fetch all sources needed for win builds
     download-linux: run 'make download-linux' to fetch all sources needed for linux builds
+    build-all: build the dependencies for all the arch/OS
 
 ### Other documentation
 

doc/build-unix.md

@@ -79,6 +79,9 @@ Then update the package list and install `cmake`:
     sudo apt update
     sudo apt install cmake
 
+Now, you can either build from self-compiled [depends](/depends/README.md) or
+install the required dependencies with the following instructions.
+
 Options when installing required Boost library files:
 
 1. On at least Ubuntu 16.04+ and Debian 9+ there are generic names for the