CI: build on Linux, Windows and MacOS via GitHubAction (#679)

* create GitHubAction to build via cmake on ubuntu

make just with the assistant

* GHA/cmake: build via matrix for different OS

* build for Linux-x86, Linux-arm and MacOS
* code taken from https://github.community/t/create-matrix-with-multiple-os-and-env-for-each-one/16895/6

* GHA: add a build on Windows

.github/workflows/cmake-linux.yml

@@ -0,0 +1,64 @@
+name: CMake
+
+on: [push]
+
+env:
+  # Customize the CMake build type here (Release, Debug, RelWithDebInfo, etc.)
+  BUILD_TYPE: Release
+
+jobs:
+  build:
+    # The CMake configure and build commands are platform agnostic and should work equally
+    # well on Windows or Mac.  You can convert this to a matrix build if you need
+    # cross-platform coverage.
+    # See: https://docs.github.com/en/free-pro-team@latest/actions/learn-github-actions/managing-complex-workflows#using-a-build-matrix
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        include:
+          - os: ubuntu-latest
+            TARGET: x86_64-unknown-linux-musl
+            COMPILER: gcc
+            LINKER: gcc
+          - os: ubuntu-latest
+            TARGET: armv7-unknown-linux-musleabihf
+            COMPILER: arm-linux-gnueabihf-gcc-5
+            LINKER: gcc-5-arm-linux-gnueabihf
+          - os: windows-latest
+            COMPILER: cl
+            LINKER: cl
+          - os: macos-latest
+            TARGET: x86_64-apple-darwin
+            COMPILER: clang
+            LINKER: clang
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Create Build Environment
+      # Some projects don't allow in-source building, so create a separate build directory
+      # We'll use this as our working directory for all subsequent commands
+      run: cmake -E make_directory ${{github.workspace}}/build
+
+    - name: Configure CMake
+      # Use a bash shell so we can use the same syntax for environment variable
+      # access regardless of the host operating system
+      shell: bash
+      working-directory: ${{github.workspace}}/build
+      # Note the current convention is to use the -S and -B options here to specify source 
+      # and build directories, but this is only available with CMake 3.13 and higher.  
+      # The CMake binaries on the Github Actions machines are (as of this writing) 3.12
+      run: cmake $GITHUB_WORKSPACE -DCMAKE_BUILD_TYPE=$BUILD_TYPE
+
+    - name: Build
+      working-directory: ${{github.workspace}}/build
+      shell: bash
+      # Execute the build.  You can specify a specific target with "--target <NAME>"
+      run: cmake --build . --config $BUILD_TYPE
+
+    - name: Test
+      working-directory: ${{github.workspace}}/build
+      shell: bash
+      # Execute tests defined by the CMake configuration.  
+      # See https://cmake.org/cmake/help/latest/manual/ctest.1.html for more detail
+      run: ctest -C $BUILD_TYPE