Bug#1143707: sparsehash: FTBFS: with gcc-16

[Peter B]

Quick fix for this (avoiding patching)
is to add

export DEB_CXXFLAGS_MAINT_APPEND = -std=c++17

towards the top of the rules file.
Tested with sbuild and it builds OK.


Otherwise, see;
  https://github.com/sparsehash/sparsehash/pull/165