First time contributor here, still learning the ropes.

We're cross-compiling python in an environment where we set up CPPFLAGS, LDFLAGS, etc. to point directly to the locations where we have built Python's dependencies. For example, we will typically build Python in an environment that includes:

  CPPFLAGS=\
    -isystem /path/to/ncurses/build/include \
    -isystem /path/to/libffi/build/include \
    -isystem /path/to/zlib/build/include \
    -isystem /path/to/openssl/build/include \
    -isystem /path/to/readline/build/include
  
  LDFLAGS=\
    -L/path/to/ncurses/build/lib \
    -L/path/to/libffi/build/lib \
    -L/path/to/zlib/build/lib \
    -L/path/to/openssl/build/lib \
    -L/path/to/ciscossl-fom/build/lib \
    -L/path/to/readline/build/lib

setup.py already picks up our -L options from LDFLAGS and propagates them into the build commands, but the -isystem options from CPPFLAGS are currently ignored.

I will post a PR that teaches setup.py to handle -isystem options in CPPFLAGS the same way it currently handles -I options.