Fix installed Pidgin headers

* Install Pidgin headers to the correct directory.
The main header includes subheaders as `pidgin/...`, so they need to be installed that way.
* Add libhandy to pidgin pkgconfig.
* Ensure `pidginenums.h` is installed.
* Add some variables around pidgin include install paths.

Testing Done:
Compiled a file with `#include ` using `pkgconf --cflags pidgin-3`.

Reviewed at https://reviews.imfreedom.org/r/656/