Automated Windows cross compilation

Preliminary support for automated Windows cross compilation, using
MingGW-w64. A script is provided, with 32 and 64 bits compilation
options, along with a documentation file

git-svn-id: svn://svn.debian.org/nut/branches/windows_port@3821 72a954d1-e00c-0410-aa02-d9c7bb700a61
Fix MingGW-w64 compilation

git-svn-id: svn://svn.debian.org/nut/branches/windows_port@3822 72a954d1-e00c-0410-aa02-d9c7bb700a61
Forgotten build and distribution rules

git-svn-id: svn://svn.debian.org/nut/branches/windows_port@3823 72a954d1-e00c-0410-aa02-d9c7bb700a61