Fix macos CI builds

.github/workflows/osrm-backend.yml

@@ -462,6 +462,11 @@ jobs:
         fi
         echo "CC=${CCOMPILER}" >> $GITHUB_ENV
         echo "CXX=${CXXCOMPILER}" >> $GITHUB_ENV
+        if [[ "${RUNNER_OS}" == "macOS" ]]; then
+          # missing from GCC path, needed for conan builds of libiconv for example.
+           sudo xcode-select --switch /Library/Developer/CommandLineTools
+          echo "LIBRARY_PATH=${LIBRARY_PATH}:/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib" >> $GITHUB_ENV
+        fi
 
     - name: Build and install OSRM
       run: |