require a libjpeg implementation. jpeg.buildlink3.mk will:
* set JPEGBASE to the base directory of the libjpeg files;
* set JPEG_TYPE to the libjpeg implementation used.
There are three variables that can be used to tweak the selection of
the libjpeg implementation:
JPEG_DEFAULT is a user-settable variable whose value is the default
libjpeg implementation to use.
JPEG_ACCEPTED is a package-settable list of libjpeg implementations
that may be used by the package.
This .mk is broadly based on fam.buildlink3.mk,v 1.7, and
currently supports selection between the default "graphics/jpeg"
and the alternative "graphics/libjpeg-turbo".
