Desired Behavior

Dune should take into account the CFLAGS environment variable when compiling C code (similarly with CXXFLAGS and C++).

Rationale

Using these environment variables is standard in build systems (e.g. make).

In Debian, we use these variables to have some options used by many (if not all) packages at once.

In particular, one of the options set by Debian is the file prefix map: -ffile-prefix-map=/tmp/foo=., which is important for reproducibility.