Suppress find_package_handle_standard_args package name mismatch warning.

Summary:
cmake introduced a new find_package mismatch check in 3.17, but also allows
user to suppress the warning by setting a variable (Or parameter, but
require new cmake 3.17). Same technique is also used with in cmake,
e.g. FindGTK2.cmake.

Test Plan: Test with FindXCB.cmake

Reviewers: Frameworks, Build System, apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D29396