[MesonToolchain] Review mechanism to add C* flags (now env and conf)

[franramirez688]

I still feel this is problematic, having 2 mechanisms to define CXX_FLAGS (env and conf). Not related to this PR, I know it is already this way for C/C++ flags, but something we want to clarify for the future.

Originally posted by @memsharded in #11632 (comment)

UPDATE

MesonToolchain is initializing some flags variables through the VirtualBuildEnv:

        # Read the VirtualBuildEnv to update the variables
        build_env = VirtualBuildEnv(self._conanfile).vars()
        # ...
        self.c_args = self._get_env_list(build_env.get("CFLAGS", []))
        self.c_link_args = self._get_env_list(build_env.get("LDFLAGS", []))
        self.cpp_args = self._get_env_list(build_env.get("CXXFLAGS", []))
        self.cpp_link_args = self._get_env_list(build_env.get("LDFLAGS", []))

But also, it's possible to add more flags through the conf. I guess we could keep only the conf or think of another solution.

[franramirez688]

As it does not have a high priority and does not look critical, let's close the issue for now.