fix and refactored libzstd_partial_build.sh
fix and refactored libzstd_partial_build.sh

make the test intention clearer
(easier to maintain)

also resist the existence of zstd_ddict.o within libzstd.a

Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd