gh: add a GitHub workflow for compile+testsuite testing (#313)

This will cause all pushed commits and pull requests to be
fed through a compile and testsuite run.

.github/workflows/build.yml

@@ -0,0 +1,25 @@
+name: Compilation test
+
+on:
+  push:
+    branches: [ "master" ]
+  pull_request:
+    branches: [ "master" ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - name: install-deps
+      uses: awalsh128/cache-apt-pkgs-action@latest
+      with:
+        packages: binutils cmake doxygen g++ libcppunit-dev libgnutls28-dev libgsasl7-dev libgtk-3-dev libssl-dev make pkg-config
+        version: 1.2
+    - name: clone-vmime
+      uses: actions/checkout@v3
+    - name: buildall
+      run: |
+        set -x
+        lscpu -b --online --parse | grep -v '^#' | wc -l >/tmp/ncpus
+        cmake . -DVMIME_SENDMAIL_PATH:STRING="/usr/sbin/sendmail" -DVMIME_HAVE_TLS_SUPPORT:BOOL=ON -DVMIME_BUILD_TESTS=ON -DVMIME_BUILD_SAMPLES=ON && make "-j$(cat /tmp/ncpus)" && sudo make install
+        make test