Update min-macos to 10.9

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++
```

CMakeLists.txt

@@ -78,13 +78,14 @@ cmake_policy( SET CMP0075 NEW )
 
 # Definitions that must happen before the project() command
 if( APPLE )
-
+   set( MIN_MACOS_VERSION 10.9 )
+   set( TARGET_MACOS_VERSION 10.13 )
    # Generate schema files
    set( CMAKE_XCODE_GENERATE_SCHEME ON )
 
    # Define the OSX compatibility parameters
    set( CMAKE_OSX_ARCHITECTURES x86_64 CACHE INTERNAL "" )
-   set( CMAKE_OSX_DEPLOYMENT_TARGET 10.7 CACHE INTERNAL "" )
+   set( CMAKE_OSX_DEPLOYMENT_TARGET ${MIN_MACOS_VERSION} CACHE INTERNAL "" )
    set( CMAKE_OSX_SYSROOT macosx CACHE INTERNAL "" )
    set( CMAKE_XCODE_ATTRIBUTE_CODE_SIGN_IDENTITY "" CACHE INTERNAL "" )
 
@@ -121,6 +122,21 @@ project( Audacity )
 # Load our functions/macros
 include( AudacityFunctions )
 
+# Allow user to globally set the library preference
+cmd_option( ${_OPT}lib_preference
+            "Library preference [system (if available), local]"
+            "local"
+            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
+)
+
+include( AudacityDependencies )
+
 # Pull all the modules we'll need
 include( CheckCXXCompilerFlag )
 include( CheckIncludeFile )

linux/build-environment/build_audacity.sh

@@ -12,6 +12,7 @@ mkdir -p build
 cd build
 
 cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release \
+    -Daudacity_lib_preference=system \
     -Daudacity_obey_system_dependencies=On \
     -Daudacity_use_wxwidgets=local \
     -Daudacity_use_expat=system \

src/CMakeLists.txt

@@ -1155,7 +1155,7 @@ elseif( CMAKE_SYSTEM_NAME MATCHES "Darwin" )
    if( PLATFORM_VERSION_SUPPORTED )
       list( APPEND LDFLAGS
          PRIVATE
-            -Wl,-platform_version,macos,10.7,10.13
+            -Wl,-platform_version,macos,${MIN_MACOS_VERSION},${TARGET_MACOS_VERSION}
       )
    else()
       list( APPEND LDFLAGS