Add a framework to generate offline dependencies

`prepare_offline_dependencies.sh` will generate a tarball containing:

* Files required for PIP to install Conan
* Conan download cache

The resulting tarball content depends on CMake flags passed down
to the script.

`build_package.sh` demonstrates how to use the offline dependecies
to build Audacity. The script requires the exact CMake options that were
provided to `prepare_offline_dependencies.sh`

`build_packages.sh` allows validating the build process for different
Linux distros. It uses Docker to:

* Generate the dependencies tarball.
* Execute an offline build using `build_package.sh`.
* Generate distro specific packages, offline as well.

linux/packages/prepare_offline_dependencies.sh

@@ -0,0 +1,62 @@
+#!/usr/bin/env bash
+
+# Usage prepare_offline_environment.sh cmake-options... 
+
+set -euxo pipefail
+
+packageName="audacity-offline-dependencies"
+scriptLocation=$(dirname "$(readlink -f "$0")")
+audacityLocation=$(readlink -f "$scriptLocation/../..")
+resultsLocation=$(pwd)
+packageLocation="$resultsLocation/$packageName"
+
+echo "Audacity location: $audacityLocation"
+echo "Result location: $resultsLocation"
+
+mkdir -p $packageLocation
+
+cd $packageLocation
+
+# Create pip download cache containing all the
+# packages requiredf to istall Conan
+
+mkdir -p pip_cache
+
+pushd pip_cache
+
+    pip3 download conan
+
+popd
+
+# Cache all Conan dependencies
+
+mkdir -p temp 
+
+pushd temp
+    python3 -m venv conan_env
+
+    source conan_env/bin/activate
+    pip3 install --no-index --find-links "$packageLocation/pip_cache" conan 
+
+    export CONAN_USER_HOME="$packageLocation/conan_home"
+    mkdir -p $CONAN_USER_HOME
+
+    conan config home
+    conan config init
+    conan config set storage.download_cache="$CONAN_USER_HOME/download_cache"
+
+    # Configure Audacity so we can collect the required
+    # packages.
+
+    cmake ${@} $audacityLocation
+
+    conan remove "*" --src --builds --packages --force 
+popd
+
+rm -R temp
+
+cd $resultsLocation
+
+tar -czf "$packageName.tar.gz" $packageName
+
+rm -R $packageLocation