It looks like the SCsub for cvtt was copied from squish and it left
the `if env['build_squish']:` line intact. This means that using
`scons builtin_squish=no modules/cvtt` would fail and overal builds
would also fail because they'd fail to find `ConvectionKernels.h`
Include paths are processed from left to right, so we use Prepend to
ensure that paths to bundled thirdparty files will have precedence over
system paths (e.g. `/usr/include` should have lowest priority).
