mingw_fork_compressor() uses CreateProcess() to run the compressor program. This will often be an instance of BusyBox, but since the xv and lzma applets in BusyBox don't support compression it can be an external program. It was intended that the external program should be found using PATH. However, CreateProcess() looks in various other places before trying PATH. In particular, it first looks in the directory of the current executable, then in the current directory of the process. This can result in the wrong xz.exe or lzma.exe being found. Perform an explicit PATH search and force CreateProcess() to use the result. This change only affects the search for a compressor. The same problem also affects other uses of our popen(3) emulation. These may be addressed in future. Costs 64-80 bytes. (GitHub issue #376)