Build a minimized Nix with MinGW

At this point many features are stripped out, but this works:

- Can run libnix{util,store,expr} unit tests
- Can run some Nix commands

Co-Authored-By volth <volth@volth.com>
Co-Authored-By Brian McKenna <brian@brianmckenna.org>

src/libutil/serialise.cc

@@ -7,6 +7,11 @@
 
 #include <boost/coroutine2/coroutine.hpp>
 
+#ifdef _WIN32
+# include <fileapi.h>
+# include "windows-error.hh"
+#endif
+
 
 namespace nix {
 
@@ -126,6 +131,14 @@ bool BufferedSource::hasData()
 
 size_t FdSource::readUnbuffered(char * data, size_t len)
 {
+#ifdef _WIN32
+    DWORD n;
+    checkInterrupt();
+    if (!::ReadFile(fd, data, len, &n, NULL)) {
+        _good = false;
+        throw WinError("ReadFile when FdSource::readUnbuffered");
+    }
+#else
     ssize_t n;
     do {
         checkInterrupt();
@@ -133,6 +146,7 @@ size_t FdSource::readUnbuffered(char * data, size_t len)
     } while (n == -1 && errno == EINTR);
     if (n == -1) { _good = false; throw SysError("reading from file"); }
     if (n == 0) { _good = false; throw EndOfFile(std::string(*endOfFileError)); }
+#endif
     read += n;
     return n;
 }