Rename "cygwin" option in "mingw"
The name of the option is misleading since even under Cygwin we internally use MinGW embedded within GCC 3.
The old option should give an error message pointing to the new one.
Example mexopts.bat should be given for at least 3 cases (in the package and in the wiki):
- Cygwin + GCC 3 (hence 32-bit)
- MinGW 32-bit (probably for TDM build)
- MinGW 64-bit (probably for TDM build)