Add back CMake build config for macOS (#2729)

Co-authored-by: Andrew Calcutt <acalcutt@techidiots.net>

vendor/metal-cpp.cmake

@@ -0,0 +1,21 @@
+if(TARGET mbgl-vendor-metal-cpp)
+    return()
+endif()
+
+add_library(
+    mbgl-vendor-metal-cpp INTERFACE
+)
+
+target_include_directories(
+    mbgl-vendor-metal-cpp SYSTEM
+    INTERFACE ${CMAKE_CURRENT_LIST_DIR}/metal-cpp
+)
+
+set_target_properties(
+    mbgl-vendor-metal-cpp
+    PROPERTIES
+        INTERFACE_MAPBOX_NAME "metal-cpp"
+        INTERFACE_MAPBOX_URL "https://developer.apple.com/metal/cpp/"
+        INTERFACE_MAPBOX_AUTHOR "Apple"
+        INTERFACE_MAPBOX_LICENSE ${CMAKE_CURRENT_LIST_DIR}/metal-cpp/LICENSE.txt
+)