Added GitHub actions for CI builds

These build the default project configuration across a selection of common
platform and tool versions.

.github/workflows/linux-ci.yml

@@ -0,0 +1,28 @@
+name: Linux CI
+
+on: [workflow_dispatch, push, pull_request]
+
+jobs:
+  build:
+    runs-on: ${{matrix.os}}
+
+    strategy:
+      matrix:
+        os: [ ubuntu-22.04, ubuntu-20.04 ]
+        compiler:
+          - { cc: "gcc", cxx: "g++" }
+          - { cc: "clang", cxx: "clang++" }
+
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Configure
+      run: cmake -B ${{github.workspace}}/build
+      env:
+        CC: ${{matrix.cc}}
+        CXX: ${{matrix.cxx}}
+
+    - name: Build
+      run: cmake --build ${{github.workspace}}/build
+      env:
+        MAKEFLAGS: "-j2"

.github/workflows/macos-ci.yml

@@ -0,0 +1,22 @@
+name: macOS CI
+
+on: [workflow_dispatch, push, pull_request]
+
+jobs:
+  build:
+    runs-on: ${{matrix.os}}
+
+    strategy:
+      matrix:
+        os: [ macos-12, macos-11, macos-10.15 ]
+
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Configure
+      run: cmake -B ${{github.workspace}}/build
+
+    - name: Build
+      run: cmake --build ${{github.workspace}}/build
+      env:
+        MAKEFLAGS: "-j3"

.github/workflows/windows-ci.yml

@@ -0,0 +1,29 @@
+name: Windows CI
+
+on: [workflow_dispatch, push, pull_request]
+
+jobs:
+  build:
+    runs-on: windows-2022
+
+    strategy:
+      matrix:
+        config:
+          - { vs: "17 2022", arch: "Win32", triplet: "x86-windows" }
+          - { vs: "17 2022", arch: "x64", triplet: "x64-windows" }
+
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Set up Visual Studio shell
+      uses: egor-tensin/vs-shell@v2
+      with:
+        arch: ${{matrix.config.arch}}
+
+    - name: Configure
+      shell: cmd
+      run: cmake -B ${{github.workspace}}/build -G "Visual Studio ${{matrix.config.vs}}" -A "${{matrix.config.arch}}"
+
+    - name: Build
+      shell: cmd
+      run: cmake --build ${{github.workspace}}/build

.gitignore

@@ -1,6 +1,6 @@
 .*
 !.gitignore
-!.travis.yml
+!.github
 Thumbs.db
 /out
 /CMakeSettings.json