+dnl Note OSTYPE: On Cygwin we need to use 'cygpath' to convert /cygdrive/c/foo to c:/foo
+dnl but we must not do that if we aren't building using Cygwin (notably msys),
+dnl because cygpath doesn't exist. It seems that 'bash' sets OSTYPE to 'cygwin'
+dnl or 'msys' respectively, but cygwin's 'sh' does not. So we hackily assume
+dnl that if the shell hasn't set it to 'msys' then we must be in Cygwin. Sigh.
+dnl
+dnl The Right Thing is probably to test $BuildPlatform instead, but we are sloppy
+dnl about setting that correctly at the moment, so we just work around for now.
+dnl
+dnl The quotes round "$(OSTYPE)" are essential, for the Cygwin-sh case where OSTYPE
+dnl is not set.