Support a way to get taskchampion.h in a build script

[djmitche]

Currently, to link against the taskchampion-lib C functions, you need to copy taskchampion.h out of the source directory (or, in Taskwarrior's case, put that directory in the include path). That's going to lead to sadness when the version of taskchampion-lib and of taskchampion.h inevitably fall out of sync.

Let's provide a function or constant on taskchampion-lib that contains the header file, so that consumers of the library can put the header file wherever they want using a simple build script (build.rs).

[djmitche]

ffizz might help with this

[djmitche]

Working on this in my use-ffizz-header branch.

[djmitche]

Fixed in #3114.