Fix macOS min version and bump CI (#24)

We never actually used the MACOS_MIN_VERSION variable anywhere so it was defaulting to the version of the macOS build host.
The macos-10.15 Azure DevOps image is getting deprecated so bump to macos-12 and fix the build so it still compiles with a min version of 10.15

The configure checks need -Werror=unguarded-availability otherwise they'll erreanously detect presence of symbols

ci/common-variables.yml

@@ -31,7 +31,7 @@ variables:
   - name: LinuxPool
     value: ubuntu-18.04
   - name: MacPool
-    value: macos-10.15
+    value: macos-12
   - name: LinuxCompilerSuffix
     value: -10
   - name: BuildScriptCommonOptions

cmake/toolchain.apple.cmake

@@ -48,6 +48,7 @@ if(TARGET_PLATFORM MATCHES "^host-darwin-|^ios-|^tvos-|^catalyst-")
     set(DEPLOYMENT_TARGET ${TVOS_MIN_VERSION})
   elseif(PLATFORM MATCHES "^MACOS_")
     set(INCLUDE_IOS_TOOLCHAIN False)
+    set(CMAKE_OSX_DEPLOYMENT_TARGET ${MACOS_MIN_VERSION} CACHE STRING "Minimum OSX version")
   else()
     message(FATAL_ERROR "Platform '${PLATFORM}' not supported")
   endif()

cmake/versions.apple.cmake

@@ -1,4 +1,4 @@
 set(IOS_MIN_VERSION "10.0")
-set(MACOS_MIN_VERSION "10.9")
+set(MACOS_MIN_VERSION "10.15")
 set(MACOS_CATALYST_MIN_VERSION "13.1")
 set(TVOS_MIN_VERSION "9.0")

src/native/CMakeLists.txt

@@ -186,6 +186,15 @@ if(APPLE)
   set(HOST_MACROS
     _DARWIN_C_SOURCE
     )
+
+    # Apple platforms like macOS/iOS allow targeting older operating system versions with a single SDK,
+    # the mere presence of a symbol in the SDK doesn't tell us whether the deployment target really supports it.
+    # The compiler raises a warning when using an unsupported API, turn that into an error so check_symbol_exists()
+    # can correctly identify whether the API is supported on the target.
+    check_cxx_compiler_flag("-Wunguarded-availability" "C_SUPPORTS_WUNGUARDED_AVAILABILITY")
+    if(C_SUPPORTS_WUNGUARDED_AVAILABILITY)
+      set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror=unguarded-availability")
+    endif()
 endif()
 
 macro(_compiler_has_flag _lang _flag)