allow :include in include_dirs field

[nilsbecker]

Desired Behavior

the include_dirs subfield of the foreign_stubs field of the library stanza should accept :include files.

this would be useful when running a script to find out the necessary include dirs, to include the results of that step.

Example

i was trying to link to the python headers corresponding to the installed python executable. the directory is readily obtained from python with a rule like this:

(rule
    (target python-includes)
    (mode promote)
    (alias get-python-includes)
    (action 
        (with-stdout-to 
            python-includes 
            (run python -c "import sysconfig; print(sysconfig.get_paths()['include'])"))))

i then wanted to read this file like this:

(library 
    (name python_ocaml_init) 
    (foreign_stubs 
        (language c) 
        (names python_init) 
        (flags :standard -w -fPIC -Wall -O2)
        (include_dirs (:include python-includes)))
    (modules python_init)
[...]

which failed. i found a workaround using the flags field which does accept :include but the solution above is cleaner IMO.