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 Native MinGW File Name Conversion, and
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.