add separate yml action files for windows/linux/mac

.github/workflows/macos.yml

@@ -0,0 +1,25 @@
+name: MacOS
+
+on:
+  push:
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [macos-latest]
+        compiler: [g++, clang++]
+        flags: [-std=c++11, -std=c++17]
+        optimize: [-O2]
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2.0.0
+    - name: Build and test
+      env:
+        CXX: ${{ matrix.compiler }}
+        CXXFLAGS: ${{ matrix.flags }} ${{ matrix.optimize }}
+      run: |
+        mkdir build && cd build && cmake -DPHMAP_BUILD_TESTS=ON -DPHMAP_BUILD_EXAMPLES=ON -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON .. && cmake --build . && make test