Update the build documentation

linux/build.txt

@@ -1,92 +0,0 @@
-Instructions for Building on Linux
-
-Fuller instructions can be found on our wiki at http://wiki.audacityteam.org/wiki/Building_On_Linux.
-
-In fact you should read those instructions rather than these 
-once.  These ones give less detail.
-
-
-Some prerequisites:
-
-- Audacity requires gcc 4.9 or later to build.
-
-- CMake ( http://www.cmake.org/ ) is required to build the local copy of 
-  the libsoxr resampling library used by Audacity.
-
-On a modern Debian distro, e.g. Ubuntu 16.04 (bionic), you would do:
-
-sudo apt-get install build-essential cmake
-
-- libasound and gtk are required. Having gtk2 and gtk3 too  
-  may not be required, but compilation works if both are.
-
-- Auto-tools are also needed.
-
-sudo apt-get install libasound2-dev libgtk2.0-dev libgtk-3-dev
-sudo apt-get install autoconf automake
-
-- Since you will be fetching code from git repositories you 
-  will need git.
-
-sudo apt-get install git git-gui gitk
-
-
-wxWidgets:
-
- 1) Clone wxWidgets and checkout 3.1.3 from the Audacity fork of the 
-    wxWidgets project: 
-       https://github.com/audacity/wxWidgets/....
-
-    for example
-      mkdir ./wxWidgets
-      cd ./wxWidgets
-      git clone --recurse-submodules https://github.com/audacity/wxWidgets/
-
-    Don't be tempted to use Widgets already installed on Linux because this 
-    will typically be a different version and will cause problems.              
-
-    IF you forgot the --recurse-submodules, you can correct that later by:
-      git submodule update --init
-
-  2) Follow instructions for building at 
-    https://github.com/audacity/wxWidgets/blob/audacity-fixes-3.1.3/docs/gtk/install.md
-
-    So...
-
-    mkdir buildgtk
-    cd buildgtk
-    ../configure --with-gtk
-    make
-    su <type root password>
-    make install
-    ldconfig    
-
-  3) Having got this far, it is well worth trying out building some wxWidgets
-    examples to confirm that building has worked OK
-
-Audacity:
-
-See our wiki for more detailed build instructions, including how to 
-install CMake.
-
-
-1. Make a build directory under the audacity git directory.
-
-  cd audacity
-  mkdir build-cmake
-  cd build-cmake
-
-
-2. Generate the makefiles
-
-  cmake -DCMAKE_BUILD_TYPE=Release ..
-
-3. Build Audacity 
-
-  cmake --build . 
-
-4. Run
-
-  cd bin/Release
-  ./audacity
-