Refactor our CMake flag handling to make it more flexible and reduce
repetition:
- Rename update_cxx_flags() to update_build_flags() to better reflect
its expanded purpose
- Generate CMake variable names internally based on configuration type
instead of requiring callers to specify full variable names
- Follow CMake's standard naming conventions for configuration-specific
flags, see
https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_FLAGS.html#variable:CMAKE_%3CLANG%3E_FLAGS
- Prepare groundwork for handling linker flags in addition to compiler
flags in future changes
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#23842