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