Log Message:
Changes 10.26.3:
* pnmremap: fix rounding error where map maxval is small
  and unequal to image maxval.
* ppmtompeg: fix compile failures on "no parallel" builds
  and where prototypes don't match definitions in bitio.c.

Changes 10.26.2:
* Build: handle zlib prerequisite of tiff library.
* giftopnm: fix crash with input that has bogus color indices
  in the raster.
* giftopnm: fix crash with input that has a bogus minimum code
  length field.
* pnmtopalm: fix padding with -offset.
* pnmtopalm: fix compressed images.  use 0 compressed size
  field - not totally correct, but passable.
* palmtopnm: fix incorrect handling of some combinations of
  PALM_IS_COMPRESSED flag and compression type field.
* palmtopnm: fix totally broken RLE and scanline compression.
* palmtopnm: fix wrong check for disallowed combination of
  -transparent and -rendition.