Log Message:
Update libpgf to version 7.21.7

Pkgsrc changes:
Don't need dos2unix, the line endings have been fixed in this version.
Removed doc/Makefile.am patch, it has been applied upstream.
This version builds on compilers defaulting to C++17.
BUILDLINK_API_DEPENDS has been bumped to the new version since nothing
in pkgsrc depends on libpgf.

Changelog from readme:

Version 7.21.7, (Thu, 18 Feb 2021)
------------------------------------

1. The new version is a minor bug fix of versions 7.15.25 to 7.21.2.

2. This bug fix for ROI decoding only. This bug fix is necessary if
   several Read() and ResetStreamPos() operations are called with
   the same PGFimage object after a single Open() call.

Version 7.21.2, (Sun, 17 Jan 2021)
------------------------------------

1. The new version is a documentation and build tool update of
   version 7.19.3.

Version 7.19.3, (Tue, 15 Jan 2019)
------------------------------------

1. The new version is a minor update of version 7.15.25.

2. This version fixes a compilation bug seen when ROI support is
   disabled.

Version 7.15.32, (Thu, 6 Aug 2015)
------------------------------------

1. The new version is a minor update of version 7.15.25.

2. This version improves the reuse of CPGFImage objects for several
   decoding operations. It clarifies the usage of CPGFImage::Close()
   and CPGFImage::Destroy() by deletion of Close(). Several reading
   operations can be performed in the following way:
   Open(), Read(), GetBitmap(), ResetStreamPos(), Read(),
   GetBitmap(), ResetStreamPos(), ...
   Calling Destroy() frees all allocated ressources and reinitializes
   the object to the same state as the constructor does. This allows
   the reuse of the CPGFImage object for encoding and decoding:
   SetHeader(), ImportBitmap(), Write(), ResetStreamPos(), Destroy(),
   Open(), Read(), GetBitmap()

3. Caching or skipping of user data (meta data) while opening a PGF
   image can be controlled by a new UserdataPolicy in
   ConfigureDecoder().

Version 7.15.25, (Sat, 20 June 2015)
------------------------------------

1. This new version is a replacement of version 6.14.12.
   In case you use the ROI decoding, we strongly encourage using
   version 6.15.25 instead of an older version.

2. This version fixes some decoder bugs only seen in ROI decoding.
   ROI decoding is now also supported for Bitmap and RGB12 image modes.

3. This version introduces a new and more efficient data format for
   binary images (bitmaps).
   The new format allows ROI decoding.
   The decoder supports both the old and the new format, but ROI
   decoding works only with the new format.

4. The two reserverd bytes in PGFHeader are now used for a more
   detailled PGF version number.

5. The Visual Studio project files are in the VS12 format.