RFR: 7903164: cleanup jextract options [v2]

[Athijegannathan Sundararajan]

On Fri, 15 Apr 2022 11:56:03 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> 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.
>
> README.md line 134:
> 
>> 132: 
>> 133: If you want to pass extra options to clang compiler, you can create a file named _compile_flags.txt_ in the current directory.
>> 134: That text file can contain clang compiler options one per each line.
> 
> Suggestion:
> 
> #### Additional clang options
> 
> Users can specify additional clang compiler options, by creating a file named `compile_flags.txt` in the current folder, as described [here](https://clang.llvm.org/docs/JSONCompilationDatabase.html#alternatives).

changed as suggested

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