Adds an environment to check, how well Linux builds with system packages

A special mode `audacity_obey_system_dependencies` is added, so the build will fail if we could not build against a system package for some reason. 

The following packages are marked for the local build now:

* **wxWidgets**: Ubuntu lacks support for 3.1.3. We can't build against 3.0 branch.
* **portaudio**: there are issues, that prevent using a system version of portaudio.
* **sqlite3**: Ubuntu package is very dated; we care about the performance and stability.
* **nyquist**: Ubuntu has no package available.
* **vamp**: Ubuntu has no development package available.
* **portmixer**: Ubuntu has no package available.
* **sbsms**: Ubuntu package is very dated; we care about the performance and stability.

We use docker to create a clean build environment. Currently, `pkg-config` is used to locate the system libraries. There are few issues with `pkg-config` on Ubuntu:

* It does not work with `lame` and `portmidi`.
* The packaged files for `id3tag` and `mad` have wrong version.
We fix such cases by copying the 
into `/usr/local/lib`.

linux/build-environment/Dockerfile

@@ -0,0 +1,66 @@
+FROM ubuntu:focal
+
+LABEL maintainer="d.vedenko@audacityteam.org"
+LABEL description="A build environment to check the builds for the package maintainers"
+LABEL version="3.0"
+
+ENV TZ=Europe/London
+RUN ln -snf /usr/share/zoneinfo/$TZ /etc/localtime && echo $TZ > /etc/timezone
+
+RUN apt-get update
+
+RUN apt-get install -y \
+    build-essential \
+    python3-minimal \
+    python3-pip \
+    g++-9 \
+    libstdc++-9-dev \
+    cmake \
+    git
+
+RUN pip3 install conan
+
+RUN apt-get install -y \ 
+    zlib1g-dev \
+    libgtk2.0-dev \
+    libasound2-dev \
+    libavformat-dev \
+    libjack-jackd2-dev \
+    libexpat1-dev \
+    libmp3lame-dev \
+    libsndfile-dev \
+    libsoxr-dev \
+    portaudio19-dev \
+    libsqlite3-dev \
+    libavcodec-dev \
+    libavformat-dev \
+    libavutil-dev \
+    libid3tag0-dev \
+    libmad0-dev \
+    libvamp-hostsdk3v5 \
+    libogg-dev \
+    libvorbis-dev \
+    libflac-dev \
+    libflac++-dev \
+    lv2-dev \
+    liblilv-dev \
+    libserd-dev \
+    libsord-dev \
+    libsratom-dev \
+    libsuil-dev \
+    libportmidi-dev \
+    libportsmf-dev \
+    libsbsms-dev \
+    libsoundtouch-dev \
+    libtwolame-dev \
+    libssl-dev \
+    libcurl4-openssl-dev
+
+WORKDIR /audacity
+
+COPY ["build_audacity.sh", "/audacity/"]
+
+# pkg-config is so broken
+COPY ["pkgconfig/*", "/usr/local/lib/pkgconfig/"]
+
+CMD ["./build_audacity.sh"]