MacOS tests on Azure fail

[KrisThielemans]

Errors like

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/cmath:317:9: error: no member named 'signbit' in the global namespace
using ::signbit;

Also seen in many other places, including UCL/STIR#531 (comment)_
with hints it's a VTK bug

[KrisThielemans]

some people suggest to include <cmath> first, before anything else. e.g. https://developer.apple.com/forums/thread/110594?answerId=338792022#338792022

[KrisThielemans]

Note however that the Azure pipeline for OSX uses ITK 5.1 (see here, which will fail due to #55

[KrisThielemans]

Clarification: errors occur on the azure pipeline where we use brew install itk. There is no problem on the Travis builds, where we install ITK ourselves.

Hopefully it will disappear when brew uses ITK 5.2.0.

[KrisThielemans]

Current errors (brew install ITK 5.1) also include https://dev.azure.com/petpvc/petpvc/_build/results?buildId=26&view=logs&j=a5e52b91-c83f-5429-4a68-c246fc63a4f7&t=2dac33d1-b0ca-5feb-3c36-18b56319df50&l=766

/usr/local/include/ITK-5.1/itkMath.h:326:32: error: expected unqualified-id
  if (std::signbit(x1) != std::signbit(x2))
                               ^
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/math.h:170:5: note: expanded from macro 'signbit'
    ( sizeof(x) == sizeof(float)  ? __inline_signbitf((float)(x))

[KrisThielemans]

I've tried to run this in #84. Still a problem with ITK 5.2 and
https://dev.azure.com/petpvc/petpvc/_build/results?buildId=52&view=logs&j=a5e52b91-c83f-5429-4a68-c246fc63a4f7&t=2dac33d1-b0ca-5feb-3c36-18b56319df50 and Xcode_13.2.1. sigh.

some people suggest to include <cmath> first, before anything else. e.g. https://developer.apple.com/forums/thread/110594?answerId=338792022#338792022

that didn't work. https://dev.azure.com/petpvc/petpvc/_build/results?buildId=53&view=logs&j=a5e52b91-c83f-5429-4a68-c246fc63a4f7&t=2dac33d1-b0ca-5feb-3c36-18b56319df50

[KrisThielemans]

There were 2 problems:

• CMake did not find Qt5 from brew. I tried to use brew --prefix to resolve that but failed when using CMake@1 as I don't know how to pass-through variables between stages, see CMake cmakeArgs does not evaluate environment variables microsoft/azure-pipelines-tasks#12848 (comment). In the end I gave up and hard-wired the location
• it still failed with errors on ::signbit etc when using the brew ITK. I've now "fixed" this by building our own ITK.

This 2 fixes are in 90a3150.

Hopefully, someone will fix the ITK version of brew at some point, but at least our CI is now fine.