Update build instructions for 2.3.0

linux/build.txt

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+Instructions for Building on Linux
+
+Fuller instructions that cover:
+    * Upgrading to gcc 4.9 on older Ubuntus
+    * Optional libraries
+can be found on our wiki at http://wiki.audacityteam.org/wiki/Building_On_Linux.
+
+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 libasound 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 3.1.1 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/v3.1.1/docs/gtk/install.txt
+
+    So...
+
+    mkdir buildgtk
+    cd buildgtk
+    ../configure --with-gtk
+    make    
+
+  3) Having got this far, it is well worth trying out building some wxWidgets
+    examples to confirm that building has worked OK
+
+Audacity:
+
+  1) Create a 'build' subdirectory under Audacity and change to it.
+
+  2) Now to build Audacity itself.  In the following command -j4 means use 4 cores.
+    Using a build directory (and .. in the command) keeps source and object files
+    in separate directories.
+
+    ../configure --with-lib-preference="local system"
+
+    make -j4
+    make install  # as root