The error message say that there is no bash.exe to be found in the C:/cygwin/bin/ path.
You mention a "build.xml" so I assume that you are using Ant to do a build.
If you are using Ant from inside CygWin then you must use the CygWin conventions for paths. So you have to use simply /bin/ as path for bash.
(In this convention C:/foo/bar/ transalte to /cygdrive/c/foo/bar/ but CygWin simulate a *nix like filesistem so there is a /bin/ and a /usr/bin/ (and so on) that point to the right places. Check CygWin docs: Using Cygwin)
Do a simple check:
- into a CygWin terminal type the command
ls /bin/bash.exe : it list the executable for bash
- open a windows CMD and type the command
dir C:\cygwin\bin\bash.exe: it list the executable for bash
If the second commad fail, probably CygWin is not installed in the standard path C:/cygwin/ so do check another path like D:/cygwin/.