INSTALL: explaining a bit more the deal with librsvg.

Rust dependencies are not really making everyone happy apparently. We
don't want to make SVG support optional anymore because it is too
important, and unfortunately there doesn't seem to be any suitable
replacement SVG libs so far (or we haven't found them).

We explain this in the INSTALL file and also explain a bit how to bypass
the SVG parts for packagers really needing to do so, allowing GIMP to be
as portable as possible.

See also issue #6821.

INSTALL.in

@@ -98,6 +98,24 @@ header files installed.
   9. libpng, libjpeg, libtiff, librsvg and lcms are hard dependencies
      that can not be disabled.
 
+     There might be some issues with librsvg, based on the fact newer
+     versions are in Rust which is not buildable on all platforms. Yet
+     SVG support was deemed too important to be considered "optional"
+     for a decent graphics activity. Nevertheless a packager really
+     intent to have GIMP running on an architecture with no Rust support
+     could still:
+
+     1) easily patch out the file-svg plug-in from build system;
+     2) build GIMP with --disable-vector-icons. Ironically librsvg is
+     needed at build time for this option, in order to create PNG
+     variants of icons (making librsvg unneeded at runtime). So all it
+     takes is to have a build machine with librsvg to create the PNG
+     icons, package and deliver them for machines without librsvg.
+
+     This is the compromise we came with, i.e. officially making SVG a
+     first-class file format, yet explaining how you could ignore it if
+     you really wanted or needed to.
+
  10. For MyPaint brushes, brushlib (libmypaint) @LIBMYPAINT_REQUIRED_VERSION@ is used.
      The libmypaint repository is hosted at: