CPPFLAGS is empty with configure_make

Hi,

I'm using `configure_make` with Bazel. My toolchain currently uses the `sysroot_feature`, which passes the flag `--sysroot` to the compiler. This feature is active for `all_compile_actions`, including `ACTION_NAMES.preprocess_assemble`.

It seems however these flags are not passed on by `configure_make` to `CPPFLAGS`. In fact, I find that `CPPFLAGS` is empty, unlike `CFLAGS` and `CXXFLAGS`! Is that expected?

Start with the configure_make handling of compiler flags and compare how CFLAGS and CXXFLAGS are populated with CPPFLAGS. Determine whether the sysroot flag from the sysroot_feature is expected to appear in CPPFLAGS, and document or test the observed behavior.

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