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`.

cmake-proxies/CMakeLists.txt

@@ -5,6 +5,12 @@ cmd_option( ${_OPT}lib_preference
             STRINGS "system" "local"
 )
 
+# Special mode, that will force dependencies to the packages provided by system unless they were set to local explicitly.
+cmd_option( ${_OPT}obey_system_dependencies
+            "Use system packages to satisfy dependencies"
+            Off
+)
+
 # Required libraries
 #
 #       directory          option      symbol      req   chk   version

cmake-proxies/cmake-modules/AudacityFunctions.cmake

@@ -574,6 +574,7 @@ function( addlib dir name symbol required check )
    if( ${use} STREQUAL "system" )
       # Look them up
       pkg_check_modules( PKG_${TARGET} ${packages} )
+
       if( PKG_${TARGET}_FOUND )
          message( STATUS "Using '${name}' system library" )
 
@@ -586,6 +587,8 @@ function( addlib dir name symbol required check )
          # And add it to our target
          target_include_directories( ${TARGET} INTERFACE ${INCLUDES} )
          target_link_libraries( ${TARGET} INTERFACE ${LIBRARIES} )
+      elseif( ${_OPT}obey_system_dependencies )
+         message( FATAL_ERROR "Failed to find the system package ${name}" )
       else()
          set( ${use} "local" )
          set_property( CACHE ${use} PROPERTY VALUE "local" )