Applies `c_flags` and `cxx_flags` from nested `env` stanzas.

  $ printenv() {
  > dune printenv --profile default --field c_flags --field cxx_flags $@
  > }

  $ printenv .
  (c_flags (":standard + in ."))
  (cxx_flags (":standard + in ."))

  $ printenv src
  (c_flags
   (":standard + in ." ":standard + in src"))
  (cxx_flags
   (":standard + in ." ":standard + in src"))

  $ printenv bin
  (c_flags ("in bin"))
  (cxx_flags ("in bin"))

  $ printenv run
  (c_flags (-DTEST_C))
  (cxx_flags (-DTEST_CPP))

  $ dune exec --profile default ./run/foo.exe
  TEST_C defined.

  $ dune exec --profile default ./run/bar.exe
  TEST_CPP defined.
