[build] generate .xcconfig files with configure_file

We previously used incremental `file(WRITE ...)` commands that gradually recreated the file on every CMake invocation. This sometimes lead to Xcode parsing a partially written file, which in turn breaks building dependend targets. Instead, we're now using a templated configuration file, which ensure that the file is created in one go and hopefully reduces the race condition between CMake and Xcode's automatic project updating.

scripts/config.xcconfig.in

@@ -0,0 +1,5 @@
+// Do not edit -- generated by CMake
+
+// mbgl-core
+mbgl_core_INCLUDE_DIRECTORIES = "@mbgl_core_INCLUDE_DIRECTORIES@"
+mbgl_core_LINK_LIBRARIES = "@mbgl_core_LINK_LIBRARIES@"