Since XCode 10, Apple does not recommend building for macOS <10.9
Reason - 10.7 and 10.8 require libstdc++
We enforce libc++
```# Shouldn't cmake do this???
string( APPEND CMAKE_CXX_FLAGS " -stdlib=libc++" )```
While this generally works, it makes the proper dependency management tricky.
Compiler message for the library support:
```
clang: warning: libstdc++ is deprecated; move to libc++ with a minimum deployment target of OS X 10.9 [-Wdeprecated]
ld: library not found for -lstdc++
```
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`.
