gutenprint-lib: Add an gimp option

pkgsrc changes:
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  * gutenprint-lib provides support for some printers as an open source
    replacement for prioritary drivers. In case of platforms with low
    computation power (e.g.Raspberry Pi 1), we might want to build
    gutenprint to only provide PPD files to CUPS without the large set of
    dependencies needed for graphics/gimp.
  * As gutenprint has been initialy designed to provide printing capability
    to GIMP, this option is enabled by default.

(triaxx)