Added clang build step to CI

As found by dpgeorge, clang has slightly different warnings than GCC.
There's really no cost to running clang as an extra build step to test
for these.

.github/workflows/test.yml

@@ -292,6 +292,27 @@ jobs:
       - name: test-valgrind
         run: make test TESTFLAGS+="-k --valgrind"
 
+  # test that compilation is warning free under clang
+  clang:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - name: install
+        run: |
+          # need toml, also pip3 isn't installed by default?
+          sudo apt-get update -qq
+          sudo apt-get install -qq python3 python3-pip
+          sudo pip3 install toml
+      - name: install-clang
+        run: |
+          sudo apt-get update -qq
+          sudo apt-get install -qq clang
+          echo "CC=clang" >> $GITHUB_ENV
+          clang --version
+      # no reason to not test again
+      - name: test-clang
+        run: make test TESTFLAGS+="-k"
+
   # self-host with littlefs-fuse for a fuzz-like test
   fuse:
     runs-on: ubuntu-20.04