Standalone GN build.

build_overrides/README.md

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+# 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 BUILD.gn 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 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(crbug.com/588513): 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.