Cygwin provides a Unix emulation environment for Windows. As part of that emulation, it provides a file system mapping that presents the Windows file system in a Unix-compatible manner. Cygwin also provides a utility cygpath that can be used to convert file names and paths between the two representations. In a correctly configured Cygwin installation, cygpath is always present, and is in the PATH.
Libtool uses cygpath to convert from Cygwin (Unix-style) file names and paths to Windows format when the build platform is Cygwin and the host platform is MinGW.
When the host platform is Cygwin, but the build platform is MSYS or some Unix system, libtool also uses cygpath to convert from Windows to Cygwin format (after first converting from the build platform format to Windows format; see see Native MinGW File Name Conversion and see Unix/Windows File Name Conversion). Because the build platform is not Cygwin, cygpath is not (and should not be) in the PATH. Therefore, in this configuration the environment variable LT_CYGPATH is required. See LT_CYGPATH.