add c++11 build to actions

Author: kirkshoop

.github/workflows/rxcpp-ci.yml

@@ -16,6 +16,20 @@ jobs:
       fail-fast: false
       matrix:
         config:
+        - {
+            name: "Linux GCC 9 Debug (C++11)", artifact: "Linux.tar.xz",
+            os: ubuntu-latest,
+            build_type: Debug,
+            cc: "gcc-9", cxx: "g++-9",
+            cmake_args: "-D CMAKE_CXX_STANDARD:STRING=11"
+          }
+        - {
+            name: "Linux GCC 9 Optimised (C++11)", artifact: "Linux.tar.xz",
+            os: ubuntu-latest,
+            build_type: RelWithDebInfo,
+            cc: "gcc-9", cxx: "g++-9",
+            cmake_args: "-D CMAKE_CXX_STANDARD:STRING=11"
+          }
         - {
             name: "Linux GCC 9 Debug (C++14)", artifact: "Linux.tar.xz",
             os: ubuntu-latest,