Fix ecm_generate_pkgconfig_file compatibility with new cmake

Summary:
CMP0053 specifies that:

• Expansion of `@VAR@ reference syntax defined by the configure_file() and string(CONFIGURE)` commands is no longer performed in other contexts.

replace it with the $ variable syntax.

Test Plan: manual

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D5017