FindFreetype: use `freetype-config.cmake` if available

The official `freetype-config.cmake` provides `Freetype::Freetype` as an
`INTERFACE` library to its exported `freetype` target. This ends up
causing issues if CMake's Find module is used to define
`Freetype::Freetype` before using `freetype-config` because some, but
not all, of its defined targets are already available triggering the
generated code that detects such situations.

Instead, try to use `freetype-config` where possible and provide bridge
logic for this module's variable guarantees.