Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/windows/signals-impl.hh

@@ -0,0 +1,41 @@
+#pragma once
+///@file
+
+#include "types.hh"
+
+namespace nix {
+
+/* User interruption. */
+
+static inline void setInterrupted(bool isInterrupted)
+{
+    /* Do nothing for now */
+}
+
+static inline bool getInterrupted()
+{
+    return false;
+}
+
+inline void setInterruptThrown()
+{
+    /* Do nothing for now */
+}
+
+void inline checkInterrupt()
+{
+    /* Do nothing for now */
+}
+
+/**
+ * Does nothing, unlike Unix counterpart, but allows avoiding C++
+ */
+struct ReceiveInterrupts
+{
+    /**
+     * Explicit destructor avoids dead code warnings.
+     */
+    ~ReceiveInterrupts() {}
+};
+
+}