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.
- Based on code from @qti3e and @piscisaureus in #724 and #1125
respectively.
- TODO The DENO_BUILD_PATH env var must be supplied and must be an
absolute path, this restriction should be removed in future work.
(Use C:\deno instead of c:\deno in appveyor config because it's cloned to c:\ by clone_folder variable in .appveyor.yml. On the other hand, build directory is pointed to C:\ by $(APPVEYOR_BUILD_FOLDER) so that test targets are placed on separated partitions.)
* Add forgotten --eol argument to 'git ls-files'. Without it, in certain
edge cases, files might have their mtime restored when it shouldn't.
* Don't parse ls-files output into fields; it's unnecessary.
* Disable the pip cache dir, and stop saving it to the appveyor cache.
* Leverage the 'git -C «dir»' flag to make a script shorter.
* Fix some factually and/or grammatically incorrect comments.
* Don't update the cache after building a PR or feature branch.
* Work around 'rustup update' erroring when an update is found.
* Log deleted directories and success/failure status.
* Make build log less noisy.
