allow custom setup of CMAKE_OSX_DEPLOYMENT_TARGET

Only set CMAKE_OSX_DEPLOYMENT_TARGET when it's not set already. This change allows setting a higher value than 11.0

Change-Id: I2992697b4caf05ad5028b3b9a8e4b1cb918da0f1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/242634
Commit-Queue: Shaobo Yan <shaoboyan@microsoft.com>
Reviewed-by: Shaobo Yan <shaoboyan@microsoft.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
diff --git a/src/cmake/DawnCompilerPlatformFlags.cmake b/src/cmake/DawnCompilerPlatformFlags.cmake
index 50638e2..7249500 100644
--- a/src/cmake/DawnCompilerPlatformFlags.cmake
+++ b/src/cmake/DawnCompilerPlatformFlags.cmake
@@ -65,5 +65,5 @@
 endif ()
 
 if (TARGET_MACOS)
-    set(CMAKE_OSX_DEPLOYMENT_TARGET "11.0" CACHE STRING "Minimum macOS version" FORCE)
+    set(CMAKE_OSX_DEPLOYMENT_TARGET "11.0" CACHE STRING "Minimum macOS version")
 endif ()
\ No newline at end of file