Help future-proof GCC/Clang unified build options

The YAML uses a single `CXXFLAGS` for GCC and Clang flags, which is mostly okay because they tend to support the same flags... but sometimes GCC and Clang differ

So added `-Wno-unknown-warning -Wno-unknown-warning-option`... the first is GCC's spelling and seems to ignore only `-Wno-xxx` negative flags, the second is Clang's spelling and seems to also ignore `-Wxxx` flags, and using them both seems to work to make GCC and Clang ignore also each other's no-warning options (which is either meta or a sweet irony)

HT: @DyXel thanks for #916 (comment)

Author: hsutter

.github/workflows/build-cppfront.yaml

@@ -42,7 +42,7 @@ jobs:
     runs-on: ${{ matrix.runs-on }}
     env:
       CXX: ${{ matrix.compiler }}
-      CXXFLAGS: -std=${{ matrix.cxx-std }} -Wall -Wextra -Wold-style-cast -Wunused-parameter -Wpedantic -Werror -pthread
+      CXXFLAGS: -std=${{ matrix.cxx-std }} -Wall -Wextra -Wold-style-cast -Wunused-parameter -Wpedantic -Werror -pthread -Wno-unknown-warning -Wno-unknown-warning-option
     steps:
     - uses: actions/checkout@v3
     - name: Install compiler