CI special case for using system version of 3rd party libraries

.github/scripts/unix.sh

@@ -68,6 +68,8 @@ function configure()
       -DGTSAM_USE_QUATERNIONS=${GTSAM_USE_QUATERNIONS:-OFF} \
       -DGTSAM_ROT3_EXPMAP=${GTSAM_ROT3_EXPMAP:-ON} \
       -DGTSAM_POSE3_EXPMAP=${GTSAM_POSE3_EXPMAP:-ON} \
+      -DGTSAM_USE_SYSTEM_EIGEN=${GTSAM_USE_SYSTEM_EIGEN:-OFF} \
+      -DGTSAM_USE_SYSTEM_METIS=${GTSAM_USE_SYSTEM_METIS:-OFF} \
       -DGTSAM_BUILD_WITH_MARCH_NATIVE=OFF \
       -DBOOST_ROOT=$BOOST_ROOT \
       -DBoost_NO_SYSTEM_PATHS=ON \

.github/workflows/build-special.yml

@@ -55,6 +55,12 @@ jobs:
             version: "9"
             flag: cayley
 
+          - name: ubuntu-system-libs
+            os: ubuntu-18.04
+            compiler: gcc
+            version: "9"
+            flag: system-libs
+
     steps:
       - name: Checkout
         uses: actions/checkout@v2
@@ -126,6 +132,12 @@ jobs:
           echo "GTSAM_ROT3_EXPMAP=OFF" >> $GITHUB_ENV
           echo "GTSAM Uses Cayley map for Rot3"
 
+      -name: Use system versions of 3rd party libraries
+       if: matrix.flag == 'system'
+       run: |
+        echo "GTSAM_USE_SYSTEM_EIGEN=ON" >> $GITHUB_ENV
+        echo "GTSAM_USE_SYSTEM_METIS=ON" >> $GITHUB_ENV
+
       - name: Build & Test
         run: |
           bash .github/scripts/unix.sh -t