replace "find pre/" with "find pre" to avoid double slashes:
```
stefan at air in ~/Dev/neovim/test/includes on git:port-mch_getperm-mch_setperm+
% make --dry-run
echo GEN /sys/stat.h
mkdir -p post//sys
cc -std=c99 -E -P pre//sys/stat.h -o post//sys/stat.h
```