Fixed a few things that allow it to compile and run on Windows:

1) htonl() and htons() are declared in winsock.h on Windows and require ws2_32 to be linked in.
2) mingw does not have asprintf, so I changed the Qt GUI to use QString's sprintf() member function which should behave identically.
3) Windows Vista and 7 try to run any program with "patch" in the name with administrative privileges for backwards compatibility with legacy stuff, so I added a build step to the Qt app to embed a manifest. I also need to do the same thing with the cli app...

Makefile.win

@@ -0,0 +1,5 @@
+# Windows requires -lws2_32 for htons(), htonl(), etc.
+# Use make -f Makefile.win to use this Makefile, which adds
+# the required library to the LDFLAGS.
+LDFLAGS += -lws2_32
+include Makefile

cli/Makefile.inc

@@ -9,6 +9,6 @@ cli_clean:
 	rm -f $(ROMPATCHER_EXE)
 
 $(ROMPATCHER_EXE): $(ROMPATCHER_SRCS:%.c=%.o) $(ROMLIB_A)
-	$(CC) $(CFLAGS) -o $@ $< $(LDFLAGS) $(ROMLIB_A)
+	$(CC) $(CFLAGS) -o $@ $< $(ROMLIB_A) $(LDFLAGS)
 
 cli/%o: cli/%.c

lib/checksum.c

@@ -1,5 +1,9 @@
 #include <stdlib.h>
+#ifdef __MINGW32__
+#include <winsock.h>
+#else
 #include <arpa/inet.h>
+#endif
 #include "macrompatcher.h"
 
 RomErr GetChecksum(RomCtx *rom, uint32_t *checksum) {

lib/locate_drvr.c

@@ -1,4 +1,8 @@
+#ifdef __MINGW32__
+#include <winsock.h>
+#else
 #include <arpa/inet.h>
+#endif
 #include <stdio.h>
 #include "macrompatcher.h"
 

qtgui/RomPatcher.cpp

@@ -100,8 +100,7 @@ void RomPatcher::updateChecksumUI()
 	uint32_t cksum;
 	char *cksumstr = NULL;
 	GetChecksum(rom, &cksum);
-	asprintf(&cksumstr, "Checksum: %#x", cksum);
-	checksum->setText(cksumstr);
+        checksum->setText(QString().sprintf("%#x", cksum));
 	free(cksumstr);
 }
 

qtgui/RomPatcher.manifest

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
+  <trustInfo xmlns="urn:schemas-microsoft-com:asm.v3">
+    <security>
+      <requestedPrivileges>
+        <requestedExecutionLevel level="asInvoker" />
+      </requestedPrivileges>
+    </security>
+  </trustInfo>
+</assembly>

qtgui/RomPatcher.pro

@@ -4,3 +4,8 @@ HEADERS = RomPatcher.h
 CONFIG += qt
 
 LIBS += ../lib/libmacrom.a
+
+win32 {
+    QMAKE_PRE_LINK += "windres --input ../qtgui/RomPatcher.rc --output RomPatcher.res --output-format=coff"
+    LIBS += -lws2_32 RomPatcher.res
+}

qtgui/RomPatcher.rc

@@ -0,0 +1,2 @@
+#include "winuser.h"
+1 RT_MANIFEST RomPatcher.manifest