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/pkgconfig/portmidi.pc

@@ -0,0 +1,11 @@
+prefix=/usr
+exec_prefix=${prefix}
+libdir=${exec_prefix}/lib/x86_64-linux-gnu
+includedir=${prefix}/include
+
+Name: portmidi
+Description: library for real-time MIDI input/output
+Requires:
+Version: 0.1
+Libs: -L${libdir} -lportmidi
+Cflags: -I${includedir}