Issue #7956: Add full BigTiff open/export support to GIMP.

Recent libtiff supports loading BigTiff automatically so we didn't have
anything to do there (as long as a recent libtiff was used). For
creating a BigTIFF though, we simply needed to add a "8" flag to
TIFFOpen/TIFFClientOpen when creating a new image (i.e. using "w8"
mode) as explained here in the "Implementation Strategy" section:
http://www.simplesystems.org/libtiff/BigTIFFProposal.html

What this commit does:

- Explicitly bump our libtiff requirement to version 4.0.0 or higher
  (which is where BigTiff support appeared).
  libtiff 4.0.0 was apparently released on 2011-12-22 and is available
  on all current distributions, so it's probably not a problem.
- Switch to detect libtiff with a pkg-config test (added in libtiff
  commit faf5f3eb before 4.0.0 release, so it's fine) instead of
  function checks.
  (Note: meson was already detecting for libtiff-4 with pkg-config,
  which was obviously wrong since it should have mimicked autotools, but
  well… then changes were minimal on meson)
- Add a new "bigtiff" boolean argument to the "file-tiff-save" PDB
  procedure, FALSE by default. I set this as the first argument as I
  figure that choosing the format you want is quite a major choice.
  Unless I misunderstood something, since BigTIFF is really designed to
  be an evolution of TIFF with a "minimum change strategy", i.e. mostly
  using 64-bit instead of 32-bit offsets, everything which is possible
  in TIFF will be in BigTIFF (and oppositely as well, except of course
  having huge files) so there is no need to have 2 separate procedures.
- Adding this new argument to the GUI dialog as a checkbox.
- Tweak the load and export procedures' documentation strings to make
  clear we support both TIFF and BigTIFF.
  Note: interestingly there doesn't seem to be a separate mimetype for
  BigTIFF so nothing to update on this side.
- Tweak the procedure labels too to mention BigTIFF. Since BigTIFF is
  still a different format (though very closely resembling) from TIFF,
  unlike some others which are just extensions embedded in a TIFF file
  (like GeoTIFF we recently added), I figure it deserves to be
  explicitly cited.

INSTALL.in

@@ -212,7 +212,7 @@ header files installed.
      libpng               @LIBPNG_REQUIRED_VERSION@
      libpoppler-glib      @POPPLER_REQUIRED_VERSION@
      librsvg              @RSVG_REQUIRED_VERSION@
-     libtiff
+     libtiff              @LIBTIFF_REQUIRED_VERSION@
      Little CMS           @LCMS_REQUIRED_VERSION@
      mypaint-brushes-1.0
      pangocairo           @PANGOCAIRO_REQUIRED_VERSION@