Cross compiling Qt with mingw on Linux

I needed to cross compile Qt for Windows using mingw (4.2.1) on Linux. Most of the support is there, but I had a few issues:

• Qmake in mingw mode created makefiles for plugins as static libs, not dlls.
• Qmake didn't delete linker scripts in distclean
• The -L option to configure didn't pass the library folder onto the config tests. This is needed by the mysql plugin '-plugin-sql-mysql'
• The project file for the textcodecs is missing some files for win32
• There's a rudimentary mkspecs, but it doesn't work.

I fixed these issues and have attached a diff of the changes. It's not large and hope it can be of help to others.