Build overrides in GN

This directory is used to allow different products to customize settings for repos which are DEPS'ed in or shared.

For example: V8 can be built on its own (in a “standalone” configuration), and it can be built as part of Chromium. V8 defines a top-level target, //v8:d8 (a simple executable), which will only be built in the standalone configuration. To indiate itis a standalone configuration, v8 can create a file, build_overrides/v8.gni, containing a variable, build_standalone_d8 = true and import it (as import(“//build_overrides/v8.gni”) from its top-level file.

Chromium, on the other hand, does not need to build d8, and so it would create its own build_overrides/v8.gni file, and in it set build_standalone_d8 = false.

The two files should define the same set of variables, but the values may vary as appropriate to suit the needs of the two different builds.

The build.gni file provides a way for projects to override defaults for variables used in //build itself (which we want to be shareable between projects).

TODO( Ideally //build_overrides and, in particular, //build_overrides/build.gni will go away completely in favor of some mechanism that can re-use other required files like //.gn, so that we don't have to keep requiring projects to create a bunch of different files to use GN.