cargo: build in Cargo's out dir if DENO_BUILD_PATH is not set

Plus some minor improvements and clean-ups:

* Resolve DENO_BUILD_PATH to an absolute path if necessary.
* Rename DENO_BUILD_PATH to GN_OUT_DIR in places where it is supposed to
  be set by the build system (and not a user configuration variable).
* Output Cargo `rerun-if-*-changed` instructions first, so even if the
  build itself fails, configuration changes will still trigger a re-run
  of build.rs.
* Remove TODOs that are impossible.
* Re-run build.rs when the flatbuffer definition file changes.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/msg.rs b/src/msg.rs
index 5c0244509..fcb10fff5 100644
--- a/src/msg.rs
+++ b/src/msg.rs
@@ -1,6 +1,6 @@
 #![allow(unused_imports)]
 #![allow(dead_code)]
 use flatbuffers;
-// TODO Replace DENO_BUILD_PATH with OUT_DIR. gn/ninja should generate into
-// the same output directory as cargo uses.
-include!(concat!(env!("DENO_BUILD_PATH"), "/gen/msg_generated.rs"));
+// GN_OUT_DIR is set either by build.rs (for the Cargo build), or by
+// build_extra/rust/run.py (for the GN+Ninja build).
+include!(concat!(env!("GN_OUT_DIR"), "/gen/msg_generated.rs"));