OS X libomp CI fix. (#5771)

* mac os libomp testing

* building from source

* fixed libomp installation

* added note to develop docs about workaround

build/ci/job-template.yml

@@ -63,7 +63,7 @@ jobs:
         continueOnError: true
     # Extra MacOS step required to install OS-specific dependencies
     - ${{ if eq(parameters.pool.name, 'Hosted macOS') }}:
-      - script: brew update && brew install mono-libgdiplus && brew install $(Build.SourcesDirectory)/build/libomp.rb && brew link libomp --force
+      - script: brew update && brew install mono-libgdiplus && brew install $(Build.SourcesDirectory)/build/libomp.rb --build-from-source --formula && brew link libomp --force
         displayName: Install MacOS build dependencies
     - ${{ if and( eq(parameters.nightlyBuild, 'true'), eq(parameters.pool.name, 'Hosted Ubuntu 1604')) }}:
       - bash: echo "##vso[task.setvariable variable=LD_LIBRARY_PATH]$(nightlyBuildRunPath):$LD_LIBRARY_PATH"

build/vsts-ci.yml

@@ -55,7 +55,7 @@ phases:
       rm -rf /usr/local/bin/2to3
     displayName: MacOS Homebrew bug Workaround
     continueOnError: true
-  - script: brew update && brew unlink python@3.8 && brew install mono-libgdiplus && brew install $(Build.SourcesDirectory)/build/libomp.rb && brew link libomp --force
+  - script: brew update && brew unlink python@3.8 && brew install mono-libgdiplus && brew install $(Build.SourcesDirectory)/build/libomp.rb --build-from-source --formula && brew link libomp --force
     displayName: Install build dependencies
   - script: ./restore.sh
     displayName: restore all projects

docs/building/unix-instructions.md

@@ -56,3 +56,5 @@ $ brew update && brew install cmake https://raw.githubusercontent.com/dotnet/mac
 ```
 
 Please note that newer versions of Homebrew [don't allow installing directly from a URL](https://github.com/Homebrew/brew/issues/8791). If you run into this issue, you may need to download libomp.rb first and install it with the local file instead.
+
+Also, libomp version 7.0.0 doesn't have a cask for Big Sur. You can work around this by downloading the libomp.rb file and then calling `brew install libomp.rb --build-from-source --formula`.