RFR: 7903837: apidiff: build.sh issues on Windows [v2]

[Jonathan Gibbons]

> Please review some `build.sh` and `Makefile` changes to address issues building on Windows.
> 
> The primary Windows issue was missing use of `mixed_path` when passing some filenames into `make`.
> There is some additional cleanup (for all platforms) to normalize the `DEPS_DIR` and add missing items to the `sanity` and `check_build_vars` targets.
> 
> This has been tested on macOS. Can someone with access to a Windows machine test it on Windows, please?

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  address review feedback

-------------

Changes:
  - all: https://git.openjdk.org/apidiff/pull/12/files
  - new: https://git.openjdk.org/apidiff/pull/12/files/61a3c05c..25765982

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=apidiff&pr=12&range=01
 - incr: https://webrevs.openjdk.org/?repo=apidiff&pr=12&range=00-01

  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/apidiff/pull/12.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/12/head:pull/12

PR: https://git.openjdk.org/apidiff/pull/12