attempt clang support

.github/workflows/ci.yml

@@ -2,13 +2,16 @@ name: CI
 run-name: continuous integration
 on: [push]
 jobs:
-  gcc:
-    runs-on: ubuntu-22.04
+  test:
+    strategy:
+      matrix:
+        os: [ubuntu-20.04, ubuntu-22.04, ubuntu-latest]
+        cc: [gcc, clang]
+    runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v3
       - run: ./tests/test.sh
         shell: bash
-        env:
-          CC: gcc
-          CXX: g++
-          FC: gfortran
+        with:
+          env:
+            CC: ${{ matrix.cc }}