support building on Windows using MinGW

Here is a patch for getting Avian to build in Windows. I used MinGW
and MSYS for the build environment. The patch has the following
changes:

1. java-nio.cpp: Had to add an include and remove a function
declaration that was reported as duplicate.
2. readme.txt: Updated the instructions to include notes about
building on Windows.
3. makefile: Added a conditional for MinGW. I left the existing "ifeq
($(platform),windows)" conditional alone so as to not break
cross-compiled Windows builds. There are some similarities between the
two, so it might be possible to combine portions of them in an elegant
manner. Since I'm not sure how the cross-compiled builds have been
done, I didn't want to mess with that portion of the makefile.

classpath/java-nio.cpp

@@ -14,6 +14,7 @@
 #include <stdlib.h>
 #include <unistd.h>
 
+#include "machine.h"
 #include "jni.h"
 #include "jni-util.h"
 
@@ -35,8 +36,6 @@
 typedef int socklen_t;
 #endif
 
-inline void* operator new(size_t, void* p) throw() { return p; }
-
 namespace {
 
 inline jbyteArray