mgwport files for curl and libidn

Using these mgwport files I was able to compile git inside mingwGitDevEnv
using the MSYSGIT make target, e. g. 
```sh
touch ../THIS_IS_MSYSGIT && make
```

These are my first mgwport files, so be careful ;)
The libexpat library was also required for building git as I found out.

Should I also create two xml files a la mingw32-tcl.xml for getting this merged?