Skip -isystem test with Bullseye Coverage tools

The Bullseye "g++" compiler does not appear to honor -isystem and
still reports -Wunused-variable messages.
try_compile: Escape CMAKE_<lang>_FLAGS in test projects (#14268)

If CMAKE_<lang>_FLAGS contains quotes or other CMake language characters
they must be escaped when written into the generated CMakeLists.txt file
so that the test project parses them properly.

Teach the TryCompile test to cover this case by adding a flag with
quotes into CMAKE_C_FLAGS during a C language try_compile.
Merge topic 'tid-system-argument' into next

538805b Skip -isystem test with Bullseye Coverage tools
Merge topic 'try_compile-escape-flags' into next

290857b try_compile: Escape CMAKE_<lang>_FLAGS in test projects (#14268)