Created attachment 1245929 [details]
build.log from scratch build

ui/tools/flood-tool.cpp: In function 'bool Inkscape::UI::Tools::compare_pixels(guint32, guint32, guint32, guint32, int, Inkscape::UI::Tools::PaintBucketChannels)':
ui/tools/flood-tool.cpp:235:49: error: call of overloaded 'abs(guint32)' is ambiguous
             return abs(static_cast<int>(ac) - ao) <= threshold;
                                                 ^

This is due to a change in GCC 7's C++ headers which makes more overloads of std::abs visible, and so calls with an unsigned argument type are ill-formed in more cases than for GCC 6 (see https://gcc.gnu.org/gcc-6/porting_to.html#overloaded-abs notes for more info).

In the code above the expression does abs((int)ac - (unsigned)ao), and the result of that is unsigned.

Presumably the intention was abs((int)ac - (int)ao), in which case another cast is needed, but I'm not going to second guess the authors.