Add the first Conan-based dependecies

add_conan_lib cmake function is defined, that allows to add a dependency using Conan with two possible system fallbacks:

1. pkg_check_modules is invoked, if `PGK_CONFIG ...` is present
2. find_package is invoked if `FIND_PACKAGE_OPTIONS` is present and `pkg_check_modules` has failed

If `ALWAYS_ALLOW_CONAN_FALLBACK` is present - `obey_system_dependencies` will be ignored for the package

Currently, the following dependencies are retrieved using Conan:

* zlib
* expat
* wxwidgets
* libmp3lame
* libid3tag
* libmad

The last three libraries are included in this commit, as they depend on zlib.
Properly pass **arch** and **os.version** to Conan

linux/build-environment/Dockerfile

@@ -56,6 +56,8 @@ RUN apt-get install -y \
     libssl-dev \
     libcurl4-openssl-dev
 
+RUN apt-get install -y libpng-dev
+
 WORKDIR /audacity
 
 COPY ["build_audacity.sh", "/audacity/"]

linux/build-environment/build_audacity.sh

@@ -38,6 +38,12 @@ cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release \
     -Daudacity_use_twolame=system \
     ../audacity
 
+exit_status=$?
+
+if [ $exit_status -ne 0 ]; then
+    exit $exit_status
+fi
+
 make -j`nproc`
 
 cd bin/Release