Fix get_include for nonstandard prefixes

[knedlsepp]

This fixes #1425.

[anntzer]

@knedlsepp Perhaps the correct solution is to locate the header relative to the actual installation, using e.g. https://importlib-metadata.readthedocs.io/en/latest/ (which will move to the stdlib in Py3.7)?
Does something like

header, = (file for file in importlib_metadata.files("pybind11")
           if file.name == "pybind11.h")
header.locate().resolve().parents[1]

print the correct include path for your setup?

[henryiii]

This was fixed, see issue.