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/current-process.hh

@@ -2,7 +2,10 @@
 ///@file
 
 #include <optional>
-#include <sys/resource.h>
+
+#ifndef _WIN32
+# include <sys/resource.h>
+#endif
 
 #include "types.hh"
 
@@ -14,16 +17,18 @@ namespace nix {
  */
 unsigned int getMaxCPU();
 
+#ifndef _WIN32 // TODO implement on Windows, if needed.
 /**
  * Change the stack size.
  */
 void setStackSize(rlim_t stackSize);
+#endif
 
 /**
  * Restore the original inherited Unix process context (such as signal
  * masks, stack size).
 
- * See startSignalHandlerThread(), saveSignalMask().
+ * See unix::startSignalHandlerThread(), unix::saveSignalMask().
  */
 void restoreProcessContext(bool restoreMounts = true);