add macOS define (#323)

* add macOS define

This fixes building the loader on macOS.

With this commit I am able to build the loader on macOS as part of the [openxr
rust crate](https://github.com/Ralith/openxrs). I have a fork of
[hotham-simulator](https://github.com/leetvr/hotham/wiki/Adding-the-Hotham-Simulator-to-your-development-environment)
working locally, allowing complete (albeit basic) openXR development locally on
my macbook.

* add change fragment

Author: kcking

changes/sdk/pr.323.gh.OpenXR-SDK-Source.md

@@ -0,0 +1 @@
+Loader: add -DXR_OS_APPLE define on macOS (fixes compilation on macOS)

src/CMakeLists.txt

@@ -139,6 +139,8 @@ elseif(CMAKE_SYSTEM_NAME STREQUAL "Linux")
     add_definitions(-DXR_OS_LINUX)
 elseif(ANDROID)
     add_definitions(-DXR_OS_ANDROID)
+elseif(APPLE)
+    add_definitions(-DXR_OS_APPLE)
 endif()
 
 # /EHsc (support for C++ exceptions) is default in most configurations but seems missing when building arm/arm64.