RFR: 7903164: cleanup jextract options [v2]

[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 one additional commit since the last revision:

  argValue check for starting with '-' was in wrong place.

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

Changes:
  - all: https://git.openjdk.java.net/jextract/pull/23/files
  - new: https://git.openjdk.java.net/jextract/pull/23/files/c430de91..eded0d1f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=23&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=23&range=00-01

  Stats: 10 lines in 1 file changed: 5 ins; 5 del; 0 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