vxl: fix build by using gcc6

[xeji]

Motivation for this change

upstream code incompatible w/gcc7. no newer version available.

/cc ZHF #36453

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

[GrahamcOfBorg]

Success on x86_64-linux (full log)

Attempted: vxl

Partial log (click to expand)

-- Installing: /nix/store/rnfrydxl3488ac9n3ymi74lm8405jk1c-vxl-1.17.0-nix1/share/vxl/cmake/VXLConfig.cmake
-- Installing: /nix/store/rnfrydxl3488ac9n3ymi74lm8405jk1c-vxl-1.17.0-nix1/share/vxl/cmake/VXLStandardOptions.cmake
-- Installing: /nix/store/rnfrydxl3488ac9n3ymi74lm8405jk1c-vxl-1.17.0-nix1/share/vxl/cmake/UseVXL.cmake
-- Installing: /nix/store/rnfrydxl3488ac9n3ymi74lm8405jk1c-vxl-1.17.0-nix1/share/vxl/cmake/UseVGUI.cmake
post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/rnfrydxl3488ac9n3ymi74lm8405jk1c-vxl-1.17.0-nix1
strip is /nix/store/b0zlxla7dmy1iwc3g459rjznx59797xy-binutils-2.28.1/bin/strip
stripping (with command strip and flags -S) in /nix/store/rnfrydxl3488ac9n3ymi74lm8405jk1c-vxl-1.17.0-nix1/lib 
patching script interpreter paths in /nix/store/rnfrydxl3488ac9n3ymi74lm8405jk1c-vxl-1.17.0-nix1
checking for references to /tmp/nix-build-vxl-1.17.0-nix1.drv-0 in /nix/store/rnfrydxl3488ac9n3ymi74lm8405jk1c-vxl-1.17.0-nix1...

[GrahamcOfBorg]

Success on aarch64-linux (full log)

Attempted: vxl

Partial log (click to expand)

-- Installing: /nix/store/i0p2nm8fh9l95r1rjn2kirzzzqi44bf3-vxl-1.17.0-nix1/share/vxl/cmake/VXLConfig.cmake
-- Installing: /nix/store/i0p2nm8fh9l95r1rjn2kirzzzqi44bf3-vxl-1.17.0-nix1/share/vxl/cmake/VXLStandardOptions.cmake
-- Installing: /nix/store/i0p2nm8fh9l95r1rjn2kirzzzqi44bf3-vxl-1.17.0-nix1/share/vxl/cmake/UseVXL.cmake
-- Installing: /nix/store/i0p2nm8fh9l95r1rjn2kirzzzqi44bf3-vxl-1.17.0-nix1/share/vxl/cmake/UseVGUI.cmake
post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/i0p2nm8fh9l95r1rjn2kirzzzqi44bf3-vxl-1.17.0-nix1
strip is /nix/store/lvx1acn1ig1j2km8jds5x3ggh3f2wa8v-binutils-2.28.1/bin/strip
stripping (with command strip and flags -S) in /nix/store/i0p2nm8fh9l95r1rjn2kirzzzqi44bf3-vxl-1.17.0-nix1/lib
patching script interpreter paths in /nix/store/i0p2nm8fh9l95r1rjn2kirzzzqi44bf3-vxl-1.17.0-nix1
checking for references to /build in /nix/store/i0p2nm8fh9l95r1rjn2kirzzzqi44bf3-vxl-1.17.0-nix1...