ECMGeneratePkgConfigFile: add variables used by pkg_check_modules

Summary:
If we ever end up in a situation where the generated .pc file is
actually used by cmake's pkg_check_modules, then we need the
includedir and libdir variables; pkg_check_modules ignores the
Makefile-like Libs and Cflags lines of the .pc file.

Context: craft adds a cmake buildsystem for sqlite3, which is then
found using pkgconfig in akonadi.

Test Plan: Filipe Azevedo is doing all the testing, I'm just providing patches :)

Reviewers: apol, fazevedo, cgiboudeaux

Reviewed By: fazevedo, cgiboudeaux

Subscribers: cgiboudeaux, fazevedo, kde-buildsystem, kde-frameworks-devel

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D19506