CI: Add workflow to test Meson and CMake builds

This adds a workflow using GitHub Actions which builds WPEBackend-fdo
twice: one time with CMake, and another with Meson. Both results are
checked to ensure that both build systems produce the same set of
headers (and with the same contents), and libraries with the same public
symbols.

.github/workflows/ci.yml

@@ -0,0 +1,101 @@
+---
+
+name: CI
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: ubuntu-18.04
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Install Debian Packages
+        run: |
+          sudo apt update
+          sudo apt install -y cmake flex libjson-glib-dev libxkbcommon-dev \
+            libegl1-mesa-dev libxml2-dev libxslt1-dev libyaml-dev llvm-dev \
+            libclang-dev libglib2.0-dev libepoxy-dev ninja-build
+      - name: Setup Python
+        uses: actions/setup-python@v1
+        with:
+          python-version: 3.6
+      - name: Python Package Cache
+        uses: actions/cache@v1
+        with:
+          path: ~/.cache/pip
+          key: ${{ runner.os }}-pip-${{ hashFiles('.github/workflows/ci.yml') }}
+          restore-keys: ${{ runner.os }}-pip-
+      - name: Install Python Packages
+        run: |
+          python -m pip install --upgrade pip setuptools wheel
+          HOTDOC_BUILD_C_EXTENSION=enabled pip install hotdoc meson==0.49
+      - name: Fetch libwpe
+        run: |
+          if [[ -d ~/libwpe/.git ]] ; then
+            echo 'Updating libwpe clone...'
+            cd ~/libwpe/
+            git reset --hard
+            git clean -qxdff
+            git checkout -f master
+            git pull -q
+          else
+            echo 'Cloning libwpe afresh...'
+            rm -rf ~/libwpe/
+            git clone -q https://github.com/WebPlatformForEmbedded/libwpe ~/libwpe/
+          fi
+      - name: Build and Install libwpe
+        run: |
+          mkdir ~/libwpe-build/ && cd "$_"
+          cmake ~/libwpe/ -DBUILD_DOCS=OFF \
+            -DCMAKE_INSTALL_PREFIX=${HOME}/libwpe-prefix \
+            -DCMAKE_INSTALL_LIBDIR=${HOME}/libwpe-prefix/lib
+          TERM=dumb cmake --build . --parallel $(nproc) --target install
+      - name: Meson - Configure
+        run: |
+          export PKG_CONFIG_PATH="${HOME}/libwpe-prefix/lib/pkgconfig/"
+          mkdir -p _work/meson
+          meson _work/meson/build --prefix /usr -Dbuild_docs=true
+      - name: Meson - Build
+        run: |
+          ninja -C _work/meson/build
+      - name: Meson - Install
+        run: |
+          DESTDIR="$(pwd)/_work/meson/prefix" ninja -C _work/meson/build install
+          nm -D -P _work/meson/build/libWPEBackend-fdo-1.0.so \
+            | awk '$2 == "T" || $2 == "U" {print $2 " " $1}' \
+            | sort -u > _work/meson/symbols
+          (cd _work/meson/prefix && find usr/lib -type f | sort) \
+            > _work/meson/files
+      - name: Meson - Archive Artifacts
+        uses: actions/upload-artifact@v1
+        with:
+          name: build-meson
+          path: _work/meson/prefix
+      - name: CMake - Configure
+        run: |
+          export PKG_CONFIG_PATH="${HOME}/libwpe-prefix/lib/pkgconfig/"
+          mkdir -p _work/cmake/build && cd $_
+          cmake -DCMAKE_INSTALL_PREFIX=/usr \
+            -DBUILD_DOCS=OFF -DEXPORTABLE_EGL=ON ../../..
+      - name: CMake - Build
+        run: |
+          make -C _work/cmake/build -j$(nproc)
+      - name: CMake - Install
+        run: |
+          DESTDIR="$(pwd)/_work/cmake/prefix" make -C _work/cmake/build install
+          nm -D -P _work/cmake/build/libWPEBackend-fdo-1.0.so \
+            | awk '$2 == "T" || $2 == "U" {print $2 " " $1}' \
+            | sort -u > _work/cmake/symbols
+          (cd _work/cmake/prefix && find usr/lib -type f | sort) \
+            > _work/cmake/files
+      - name: CMake - Archive Artifacts
+        uses: actions/upload-artifact@v1
+        with:
+          name: build-cmake
+          path: _work/cmake/prefix
+      - name: Check Installations
+        run: |
+          diff -u _work/{cmake,meson}/files
+          diff -u _work/{cmake,meson}/symbols
+          diff -Naur _work/{cmake,meson}/prefix/usr/include/