only treat compilation warnings as error in CI

Easier to work with the project as developer and with older (and probably
newer) compilers.

.github/workflows/linux-build.yml

@@ -57,6 +57,7 @@ jobs:
 
     - name: build
       run: |
+        export CXXFLAGS=-Werror
         cmake -E make_directory build
         cmake -E chdir build cmake \
           -DCMAKE_CXX_COMPILER=${{ matrix.cpp-compiler }} \

.github/workflows/qt5.yml

@@ -51,6 +51,7 @@ jobs:
 
     - name: build
       run: |
+        export CXXFLAGS=-Werror
         cmake -E make_directory build
         cmake -E chdir build cmake \
           -G Ninja \

.github/workflows/run-all.yml

@@ -56,6 +56,7 @@ jobs:
 
     - name: build and run
       run: |
+        export CXXFLAGS=-Werror
         ./run-linux.sh
 
     - name: code coverage summary report

CMakeLists.txt

@@ -98,7 +98,7 @@ endif()
 #
 
 if(CMAKE_CXX_COMPILER_ID MATCHES "Clang|GNU")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS_INIT} -Werror -Wall -Wextra -Wsuggest-override ${CMAKE_CXX_FLAGS}")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS_INIT} -Wall -Wextra -Wsuggest-override ${CMAKE_CXX_FLAGS}")
     # TODO: A better fix should handle ld's --as-needed flag
     if(UNIX AND NOT APPLE)
         set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -Xlinker '--no-as-needed'")