RFR: 7903164: cleanup jextract options

[Athijegannathan Sundararajan]

* added -D option for preprocessor macros
* renamed -d as --output
* added support for ./compile_flags.txt to pass extra clang options to jextract
* removed -C option

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Commit messages:
 - added test for -D option.
 - 7903164: cleanup jextract options

Changes: https://git.openjdk.java.net/jextract/pull/23/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=23&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903164
  Stats: 207 lines in 50 files changed: 120 ins; 9 del; 78 mod
  Patch: https://git.openjdk.java.net/jextract/pull/23.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/23/head:pull/23

PR: https://git.openjdk.java.net/jextract/pull/23