Allow building with IPv6 support, but it disabled by default

bitcoin-qt.pro

@@ -71,12 +71,16 @@ contains(FIRST_CLASS_MESSAGING, 1) {
     DEFINES += FIRST_CLASS_MESSAGING
 }
 
-# use: qmake "USE_IPV6=1" (enabled; default)
+# use: qmake "USE_IPV6=1" ( enabled by default; default)
+#  or: qmake "USE_IPV6=0" (disabled by default)
 #  or: qmake "USE_IPV6=-" (not supported)
 contains(USE_IPV6, -) {
 	message(Building without IPv6 support)
 } else {
-	DEFINES += USE_IPV6
+	count(USE_IPV6, 0) {
+		USE_IPV6=1
+	}
+	DEFINES += USE_IPV6=$$USE_IPV6
 }
 
 contains(BITCOIN_NEED_QT_PLUGINS, 1) {

src/init.cpp

@@ -500,6 +500,12 @@ bool AppInit2()
                 SetLimited(net);
         }
     }
+#if defined(USE_IPV6)
+#if ! USE_IPV6
+    else
+        SetLimited(NET_IPV6);
+#endif
+#endif
 
     CService addrProxy;
     bool fProxy = false;

src/makefile.linux-mingw

@@ -45,7 +45,7 @@ ifneq (${USE_UPNP}, -)
 endif
 
 ifneq (${USE_IPV6}, -)
-	DEFS += -DUSE_IPV6
+	DEFS += -DUSE_IPV6=$(USE_IPV6)
 endif
 
 LIBS += -l mingwthrd -l kernel32 -l user32 -l gdi32 -l comdlg32 -l winspool -l winmm -l shell32 -l comctl32 -l ole32 -l oleaut32 -l uuid -l rpcrt4 -l advapi32 -l ws2_32 -l mswsock -l shlwapi

src/makefile.mingw

@@ -41,7 +41,7 @@ ifneq (${USE_UPNP}, -)
 endif
 
 ifneq (${USE_IPV6}, -)
-	DEFS += -DUSE_IPV6
+	DEFS += -DUSE_IPV6=$(USE_IPV6)
 endif
 
 LIBS += -l kernel32 -l user32 -l gdi32 -l comdlg32 -l winspool -l winmm -l shell32 -l comctl32 -l ole32 -l oleaut32 -l uuid -l rpcrt4 -l advapi32 -l ws2_32 -l mswsock -l shlwapi

src/makefile.osx

@@ -110,7 +110,7 @@ endif
 endif
 
 ifneq (${USE_IPV6}, -)
-	DEFS += -DUSE_IPV6
+	DEFS += -DUSE_IPV6=$(USE_IPV6)
 endif
 
 all: bitcoind

src/makefile.unix

@@ -45,7 +45,7 @@ ifneq (${USE_UPNP}, -)
 endif
 
 ifneq (${USE_IPV6}, -)
-	DEFS += -DUSE_IPV6
+	DEFS += -DUSE_IPV6=$(USE_IPV6)
 endif
 
 LIBS+= \