Add CI using GitHub Actions

The Travis CI build in this repository no longer works but GitHub provides
its own CI/CD infrastructure which is free for public repositories, quote
from [1]:

  "GitHub Actions usage is free for standard GitHub-hosted runners in public
   repositories [...]"

The list of standard GitHub-hosted runners for public repositories can
be found at the following URL:

  https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories

[1] https://docs.github.com/en/billing/managing-billing-for-github-actions/about-billing-for-github-actions

.github/workflows/build.yml

@@ -0,0 +1,78 @@
+name: MVE GitHub Actions CI
+
+on:
+  push:
+    branches: [ "master" ]
+  pull_request:
+    branches: [ "master" ]
+
+jobs:
+  build:
+    name: ${{ matrix.platform.name }}
+    runs-on: ${{ matrix.platform.os }}
+
+    strategy:
+      fail-fast: false
+
+      matrix:
+        platform:
+        - { name: Ubuntu 20.04 GCC,   os: ubuntu-20.04, cpp_compiler: g++ }
+        - { name: Ubuntu 20.04 Clang, os: ubuntu-20.04, cpp_compiler: clang++ }
+        - { name: Ubuntu 22.04 GCC,   os: ubuntu-22.04, cpp_compiler: g++ }
+        - { name: Ubuntu 22.04 Clang, os: ubuntu-22.04, cpp_compiler: clang++ }
+        - { name: macOS 12,           os: macos-12,     cpp_compiler: clang++ }
+        - { name: macOS 13,           os: macos-13,     cpp_compiler: clang++ }
+        - { name: macOS 14,           os: macos-14,     cpp_compiler: clang++ }
+
+    env:
+      CXX: ${{ matrix.platform.cpp_compiler }}
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Install Ubuntu dependencies
+      if: runner.os == 'Linux'
+      run: |
+        sudo apt-get update
+        sudo apt-get install \
+          build-essential \
+          clang \
+          libgl-dev \
+          libgtest-dev \
+          libjpeg-turbo8-dev \
+          libomp-dev \
+          libpng-dev \
+          libqt5opengl5-dev \
+          libtiff-dev \
+          zlib1g-dev
+
+    - name: Install macOS dependencies
+      if: runner.os == 'macOS'
+      run: |
+        export HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK=1
+        brew update
+        brew install \
+          googletest \
+          jpeg-turbo \
+          libpng \
+          libtiff \
+          qt@5
+        brew link qt@5
+
+    - name: Build MVE on Linux/macOS
+      run: |
+        if [ "`uname`" = "Darwin"  ]; then
+          export NUM_CPU_CORES="`sysctl -n hw.ncpu`"
+        else
+          export NUM_CPU_CORES="`nproc`"
+        fi
+        make -j${NUM_CPU_CORES}
+        cd apps/umve
+        qmake
+        make -j${NUM_CPU_CORES}
+        cd ../..
+        make -j${NUM_CPU_CORES} test
+
+    - name: Run tests
+      run: ./tests/test
+