Make sure we can load and find our special protobuf version

Author: peternewman

.github/workflows/compilation.yml

@@ -38,4 +38,7 @@ jobs:
           CXX: ${{ matrix.cxx }}
         run: |
           PATH=/usr/local/opt/ccache/libexec:$PATH # Use ccache on Mac too
+          export LDFLAGS="-L/usr/local/opt/protobuf@3/lib"
+          export CPPFLAGS="-I/usr/local/opt/protobuf@3/include"
+          export PKG_CONFIG_PATH="/usr/local/opt/protobuf@3/lib/pkgconfig"
           bash -ex .travis-ci.sh