python -m pybind11 --includes prints include paths

[aldanor]

A very minor/cosmetic addition, makes building modules manually slightly easier:

$ c++ -I `python -m pybind11`

instead of

$ c++ -I `python -c 'import pybind11; print(pybind11.get_include())'`

The docs in #907 could also get updated if this gets accepted.

[jagerman]

I realize there's no __main__ at all at the moment, but I'd be a bit more receptive to this if it put the print statement behind an argument, so that you need to invoke something like python -m pybind11 --includes, which leads the door wide open for making it do other nifty things in the future.

[aldanor]

Agreed, good point re: --includes. Would you expect it to behave the same as python-config? (i.e., prefix the include paths with -I on Linux/macOS).

[jagerman]

That's probably the most convenient. Perhaps throw in the Python includes, too?

[aldanor]

Alright.

python -m pybind11 --includes

now prints include flags for both Python (borrowed from python3-config code) and pybind11.

(This is a first step, but it'd be nice to eventually integrate other stuff like recommended cflags (e.g. fvisibility-hidden, or exception flags on windows), ldflags (e.g. undefined dynamic_lookup on OS X), and extension-suffix -- so that we can remove python-config/python3-config from the docs.)

[aldanor]

Does this look better now; anything else to do here?

[dean0x7d]

LGTM