Every AppVeyor build on the specified branches that passes will be packaged
into a ZIP file and installer using CPack and uploaded to GitHub Releases.

Generate version number entirely in AppVeyor config, and build it into the
binary, and use it in the names of the created binary packages, and compile it
into the binaries.

(cherry picked from commit c5d8173)

See
http://help.appveyor.com/discussions/problems/4569-the-target-_convertpdbfiles-listed-in-a-beforetargets-attribute-at-c-does-not-exist-in-the-project-and-will-be-ignored for details.

(cherry picked from commit 019025d)

(cherry picked from commit ea4f172)

(cherry picked from commit 4876a55)

(cherry picked from commit 793d5f5)

Otherwise the unibuild fails at the end for non-administrator users when
it tries to write to C:\Program Files\BoxBackup.

Enable the relevant compiler warning as an error, if supported
(-Werror=undefined-bool-conversion).

http://www.gotw.ca/conv/002.htm
http://stackoverflow.com/questions/2165078/a-reference-can-not-be-null-or-it-can-be-null

(cherry picked from commit f2911ac)
(cherry picked from commit 5a50b98)