RFR: 7903755: multi header support and special syntax for header file [v2]

[Athijegannathan Sundararajan]

> jextract currently supports only one header file. If the user wants to extract more than one header file, s/he has to create a containing header that includes multiple headers and jextract the containing header.
> 
> With the current change, jextract supports more than one header file in the command line.
> When multiple headers are specified in command line, --header-class-name option is mandatory and it is enforced.
> 
> In addition to normal file names for header file names, a special syntax such as "<stdio.h>" is also supported. With this, user does not have to write long file name including directories such as /usr/include/stdio.h.
> 
> jextract generates multiple headers by generating a temporary header file. If the command line argument is of the form "<stdio.h>" then jextract generates "#include <stdio.h>" in the auto-generated containing header. If the header specified in of the normal/existing form (say foo.h), then containing header will have #include "foo.h" line.

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

 - Update doc/GUIDE.md
   
   Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>
 - Update doc/GUIDE.md
   
   Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>
 - Update doc/GUIDE.md
   
   Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>
 - Update doc/GUIDE.md
   
   Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>
 - Update doc/GUIDE.md
   
   Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>
 - Update doc/GUIDE.md
   
   Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>

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Changes:
  - all: https://git.openjdk.org/jextract/pull/249/files
  - new: https://git.openjdk.org/jextract/pull/249/files/0f0afe91..2086a6bf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=249&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=249&range=00-01

  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jextract/pull/249.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/249/head:pull/249

PR: https://git.openjdk.org/jextract/pull/249