cmake: use the current binary directory
cmake: use the current binary directory

This is not necessarily the top-level CMake directory (such as when
expat is included inside of another project's source tree directly), so
we should instead use the one relative to this `CMakeLists.txt` file.

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