redo-ifchange $src conf/zstd.cflags.rc conf/zstd.libs.rc
read ZSTD_CFLAGS < conf/zstd.cflags.rc
read ZSTD_LIBS < conf/zstd.libs.rc
printf "%s" "$CFLAGS $ZSTD_CFLAGS $LDFLAGS $ZSTD_LIBS" |
    tr " " "\n" | grep -v "^$" | sort | uniq