Use $PATH to find DLLs
Use $PATH to find DLLs

When --sysroot is not specificed, the $PATH is used to find DLLs. This is more
useful in Windows because DLLs are stored in the same folder as executables e.g.
Mingw's libs are in the Rust bin folder.

- Moved dll file/format check into new function dll_exists to avoid repetition.

equalsraf/winbundle