feat(osx): Add support for macOS 10.16, remove support for macOS 10.13

Stop specifying a specific path in cmake for the SDK, since the path differs in
10.16, and cmake will find it automatically.

Use the oldest supported SDK based on https://developer.apple.com/support/xcode/

.travis.yml

@@ -146,7 +146,7 @@ jobs:
           - /opt/build-windows/x86_64
     - stage: "macOS, AppImage and Flatpak"
       os: osx
-      osx_image: xcode9.3
+      osx_image: xcode10.2 # macOS 10.14, oldest supported SDK according to https://developer.apple.com/support/xcode/
       env: JOB=build-osx
       cache:
         timeout: 300 # seconds

CMakeLists.txt

@@ -60,15 +60,6 @@ execute_process(
   OUTPUT_VARIABLE QT_PREFIX_PATH
   OUTPUT_STRIP_TRAILING_WHITESPACE)
 
-execute_process(
-  COMMAND xcode-select -p
-  OUTPUT_VARIABLE CMAKE_OSX_SYSROOT
-  OUTPUT_STRIP_TRAILING_WHITESPACE)
-
-set(CMAKE_OSX_DEPLOYMENT_TARGET 10.12)
-set(CMAKE_OSX_SYSROOT
-  ${CMAKE_OSX_SYSROOT}/Platforms/MacOSX.platform/Developer/SDKs/MacOSX${CMAKE_OSX_DEPLOYMENT_TARGET}.sdk)
-
 project(qtox)
 
 # Instruct CMake to run moc automatically when needed.