macros: pass CFLAGS/CXXFLAGS to the processes started by cargo
    
    This is mainly targeted to the crates using `rust-cc`, where we want to
    apply system-wide compiler flags when invoking the compiler. `cc`
    supports CFLAGS and CXXFLAGS, but we never set those.
    Defining that per project is not possible, as we have no way to inherit
    any build context from a dependency package.
    
    Possibly also affects: `bindgen`, `cxx`, any other crate that invokes
    the C/C++ compiler and passes the CFLAGSS