reopen #2149: headers must be installed

[ghost]

Headers are required for those native add-ons that require an exact match for the installed software including use or non-use of system libraries, and/or do not use node-gyp. This includes, at least:

• SmartOS platform build
• wesolows/v8plus

and probably many others.

Please undo the removal of necessary (i.e., public and intended for use by addons) header files.

[ghost]

And now a colleague informs me that this is #5112. Thanks for shitty search, GitHub!