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.cc

@@ -19,7 +19,9 @@
 # include "namespaces.hh"
 #endif
 
-#include <sys/mount.h>
+#ifndef _WIN32
+# include <sys/mount.h>
+#endif
 
 namespace nix {
 
@@ -57,6 +59,7 @@ unsigned int getMaxCPU()
 //////////////////////////////////////////////////////////////////////
 
 
+#ifndef _WIN32
 rlim_t savedStackSize = 0;
 
 void setStackSize(rlim_t stackSize)
@@ -79,16 +82,20 @@ void setStackSize(rlim_t stackSize)
         }
     }
 }
+#endif
 
 void restoreProcessContext(bool restoreMounts)
 {
+    #ifndef _WIN32
     unix::restoreSignals();
+    #endif
     if (restoreMounts) {
         #if __linux__
         restoreMountNamespace();
         #endif
     }
 
+    #ifndef _WIN32
     if (savedStackSize) {
         struct rlimit limit;
         if (getrlimit(RLIMIT_STACK, &limit) == 0) {
@@ -96,6 +103,7 @@ void restoreProcessContext(bool restoreMounts)
             setrlimit(RLIMIT_STACK, &limit);
         }
     }
+    #endif
 }