[AdeshDeshmukh]

This pull request modernizes ghostscript-printer-app to build cleanly against CUPS 3.x and PAPPL 2.x, following the pattern reviewed and approved in OpenPrinting/ps-printer-app#41.

Summary of Changes:

1. Makefile Modernization:

   • Prioritizes pkg-config detection for cups3, falling back to cups (CUPS 2.5) and legacy cups-config when available.
   • Adds PAPPL 2.x (pappl2) detection with fallback to pappl.
   • Derives cupsserverbin cleanly via pkg-config --variable=prefix when using modern CUPS.

2. API Alignment (ghostscript-printer-app.c):

   • Updated cupsArrayNew() calls for spooling_conversions, stream_formats, and driver_selection_regex_list to the 6-argument signature (cupsArrayNew(NULL, NULL, NULL, 0, NULL, NULL)).
   • Preserves full backward compatibility with CUPS 2.x while resolving compilation failures under libcups3.

Fixes #28

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