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.
Cc: @ryan.krattiger1