Brew install libomp 7.0.0 only. (#3721)

* Brew install libomp 7.0.0 only.

* Brew install libomp 7.0.0 only.

* Brew install libomp 7.0.0 only.

* Brew install libomp 7.0.0 only.

Author: codemzs

build/ci/phase-template.yml

@@ -34,7 +34,7 @@ phases:
       ${{ insert }}: ${{ parameters.queue }}
     steps:
     - ${{ if eq(parameters.queue.name, 'Hosted macOS') }}:
-      - script: brew update && brew install libomp && brew install mono-libgdiplus gettext && brew link gettext --force && brew link libomp --force
+      - script: brew update && brew install https://raw.githubusercontent.com/Homebrew/homebrew-core/f5b1ac99a7fba27c19cee0bc4f036775c889b359/Formula/libomp.rb && brew install mono-libgdiplus gettext && brew link gettext --force && brew link libomp --force
         displayName: Install build dependencies
     - script: $(_buildScript) -$(_configuration) -buildArch=$(_arch)
       displayName: Build

build/vsts-ci.yml

@@ -48,7 +48,7 @@ phases:
     demands:
     - agent.os -equals Darwin
   steps:
-  - script: brew update && brew install libomp && brew link libomp --force
+  - script: brew update && brew install https://raw.githubusercontent.com/Homebrew/homebrew-core/f5b1ac99a7fba27c19cee0bc4f036775c889b359/Formula/libomp.rb && brew link libomp --force
     displayName: Install build dependencies
   # Only build native assets to avoid conflicts.
   - script: ./build.sh -buildNative -$(BuildConfig) -skipRIDAgnosticAssets

docs/building/unix-instructions.md

@@ -43,12 +43,12 @@ macOS 10.12 (Sierra) or higher is needed to build dotnet/machinelearning.
 
 On macOS a few components are needed which are not provided by a default developer setup:
 * cmake 3.10.3
-* libomp
+* libomp 7
 * libgdiplus
 * gettext
 * All the requirements necessary to run .NET Core 2.0 applications. To view macOS prerequisites click [here](https://docs.microsoft.com/en-us/dotnet/core/macos-prerequisites?tabs=netcore2x).
 
 One way of obtaining CMake and other required libraries is via [Homebrew](https://brew.sh):
 ```sh
-$ brew install cmake libomp mono-libgdiplus gettext && brew link gettext --force
+$ brew install cmake https://raw.githubusercontent.com/Homebrew/homebrew-core/f5b1ac99a7fba27c19cee0bc4f036775c889b359/Formula/libomp.rb mono-libgdiplus gettext && brew link gettext --force
 ```