RFR: 7903164: cleanup jextract options [v3]

[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

Athijegannathan Sundararajan has updated the pull request incrementally with two additional commits since the last revision:

 - deleted newline in README.md
 - README.md change for additional clang options (review suggestion)

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Changes:
  - all: https://git.openjdk.java.net/jextract/pull/23/files
  - new: https://git.openjdk.java.net/jextract/pull/23/files/eded0d1f..d3448912

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=23&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=23&range=01-02

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 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