Set target properties *_POSTFIX instead of global vars CMAKE_*_POSTFIX
Set target properties *_POSTFIX instead of global vars CMAKE_*_POSTFIX

This fixes unwanted side effects when libexpat is used with FetchContent
in another cmake-based project. Setting CMAKE_*_POSTFIX affects the
whole project, while here we only want to define the postfix for libexpat
itself.

libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat