Add GitHub Action to build source when pushing

Let's at least ensure that the source builds without warnings.

The build image used, Ubuntu, spits out quite some warnings about
ignored return values. Use '-Wno-unused-result' for now.

The configure options are chosen to resemble the default Debian version.

.github/workflows/build.yml

@@ -0,0 +1,26 @@
+name: ci
+
+on:
+  push:
+    branches:
+      - '*'
+    pull_request:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: install dependencies
+        run: |
+          sudo apt update
+          sudo apt install build-essential libgc-dev libgpm-dev
+
+      - name: configure
+        run: ./configure --enable-alarm --enable-ansi_color --enable-color --enable-cookie --enable-external_uri_loader --enable-gopher --enable-image --enable-ipv6 --enable-m17n --enable-menu --enable-mouse --enable-nls --enable-nntp --enable-sslverify --enable-unicode --enable-w3mmailer --with-imagelib=imlib2 --with-migemo="cmigemo -q -d /usr/share/cmigemo/utf-8/migemo-dict" --with-ssl --with-gc
+
+      - name: build
+        run: OPTS='-Wno-unused-result -Werror' make