add github action to build on several linux platforms

This github action runs a build on each of the following platforms on a
push or pull request.

- Ubuntu LTS with gcc and glibc
- Ubuntu LTS with clang and glibc
- Alpine with gcc and musl

This fixes #463.

.github/workflows/ci_meson.yml

@@ -0,0 +1,55 @@
+# Nabbed from https://github.com/mesonbuild/meson/blob/master/docs/markdown/Continuous-Integration.md#github-actions
+name: ci_meson
+
+on: [push, pull_request]
+
+jobs:
+
+  linux-gcc-glibc-ubuntu:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - run: sudo apt-get update -q
+    - run: sudo apt-get install -q -y build-essential libpam-dev meson
+    - run: meson setup builddir/
+      env:
+        CC: gcc
+    - run: ninja -C builddir
+      env:
+        CC: gcc
+
+
+  linux-clang-glibc-ubuntu:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - run: sudo apt-get update -q
+    - run: sudo apt-get install -q -y build-essential clang libpam-dev meson
+    - run: meson setup builddir/
+      env:
+        CC: clang
+    - run: ninja -C builddir
+      env:
+        CC: clang
+
+
+  linux-gcc-alpine-musl:
+    runs-on: ubuntu-latest
+    container: alpine:latest
+    steps:
+      - name: install deps
+        run: >-
+          apk --no-cache add \
+            build-base \
+            meson \
+            pkgconf \
+            linux-pam \
+            linux-pam-dev
+      - name: checkout
+        uses: actions/checkout@v2
+      - run: meson setup builddir/
+        env:
+          CC: gcc
+      - run: ninja -C builddir
+        env:
+          CC: gcc