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And followup again.

And followup again.
Here is the patch:

So, if in your copy of libraw/internal/dcraw_common.cpp
vng_interolate() code starts with:

  struct interpolate_terms {
    signed char y1, x1, y2, x2, weight;
    unsigned char grads;

than your libraw copy is indeed patched by this wrong patch.
Do not use VNG (and use patched library), or do not use gcc6 (and use unpatched library), or wait for autumn LibRaw-0.18 public snapshot (or, may be, 0.18-release)

-- Alex Tutubalin @LibRaw LLC