Try to treat warnings as errors in Boost_Tests - on Linux and Mac

[claremacrae]

Step 1:

In ef768e3d68bb4b48bd87978ddb0a7b6aa02a5dd1 ("t Try enabling Warnings-as-Errors in Boost_Tests") I turned on treating warnings as errors. The result was:

• Travis:
  • GCC failed
  • Clang passed
  • XCode passed
• Appveyor:
  • Cygwin passed
  • MinGW failed
• GitHub CI:
  • Window-latest clang-cl failed
  • Windows-2016 failed
  • all others passed

Step 2:

In ae44e3818654dae3f6193fc4cf6571fea3e8be0f ("e Treat Boost headers as system headers, to suppress warnings for them"):

• Travis:
  • All passed
• Appveyor:
  • Cygwin failed to build
  • MinGW passed
• GitHub CI:
  • Window-latest clang-cl failed
  • Windows-2016 failed
  • all others passed

[claremacrae]

Options:

• Revert the changes - just don't treat warnings as errors for Boost_Tests
• Try to fix the Cygwin build failure - it looks like Cygwin gcc cannot find stdio.h
  • Try the cygcheck examples - is /usr/include missing, or does it need to be appended to the include path?
• If I can fix the Cygwin build problem, then only turn warnings-as-errors on for Linux and Mac, and not Windows - which will at least give us some useful feedback.

[claremacrae]

This is now fixed. In Boost_Tests, warnings are treated as errors on Mac and Linux builds. On Windows, they are not.