upgraded to C++17

Author: fbourgin

.github/workflows/linux.yml

@@ -96,7 +96,7 @@ jobs:
         if [[ "${{matrix.sys.compiler}}" = "gcc" ]]; then export CC=gcc-${{matrix.sys.version}}; export CXX=g++-${{matrix.sys.version}}; else export CC=clang; export CXX=clang++; fi
         mkdir ~/.R
         touch ~/.R/Makevars
-        echo "CXX14=$CXX" >> ~/.R/Makevars
-        echo "CXX14FLAGS=-fPIC -O2" >> ~/.R/Makevars
+        echo "CXX17=$CXX" >> ~/.R/Makevars
+        echo "CXX17FLAGS=-fPIC -O2" >> ~/.R/Makevars
         Rscript ./unittest.R