Build: fix over-eager use of targets

[lgritz]

Background: Old style CMake was to use FindFoo.cmake modules, which
set FOO_INCLUDES and FOO_LIBRARIES variables. Best new cmake practices
are for the upstream packages themselves to export a FooConfig.cmake
and friends, which set up cmake targets. Of course, many packages don't
provide exported configs, so we use the old Find modules for many of
our dependencies.

I've slowly been migrating our Find modules to set up targets in addition
to the variables, and favoring use of the targets over the variables.

But I was wrong about this. It's more than just a choice of style.
When targets are used, use of transitive linkage (especially for static
library versions of dependencies) make the dependency targets show up in
our own exported OpenImageIOConfig.cmake. But to a downstream package,
now our config talks about a target like Libsquish::Libsquish, but it
doesn't know what that is, and it doesn't have the FindLibsquish.cmake
module that we used (internally) when we found that dependency.

This was causing particular problems for Libsquish on Windows with
vcpkg, but I see now that this could be a problem elsewhere as well:
field3d, libheif, openvdb, and webp.

So this patch:

• Reverts from the ill-advised targets back to the old style
  variables, for the dependencies where we use custom Find modules.
  (We presume that a dependency's exported configs, as well as any
  Find modules that are distributed as part of CMake itself and define
  targets, are done correctly, and will be available to downstream
  packages, and so we continue to use targets in those cases.)

• Beefs up our FindLibsquish.cmake module to correctly find debug
  versions of the library, to use in debug builds.

• Alters build_webp.bash to allow WEBP_BUILD_TYPE to override the build
  type, which had previously been hard-coded to Release.

• Removes search for HDF5, which as far as I can tell was unused
  directly by OIIO.

[JackBoosY]

Thanks for your fix.