Make IPv6 support buildtime-optional again (defaults to enabled)

src/makefile.mingw

@@ -3,6 +3,7 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 USE_UPNP:=0
+USE_IPV6:=1
 
 INCLUDEPATHS= \
  -I"C:\boost-1.49.0-mgw" \
@@ -23,7 +24,7 @@ LIBS= \
  -l ssl \
  -l crypto
 
-DEFS=-DWIN32 -D_WINDOWS -DBOOST_THREAD_USE_LIB -DBOOST_SPIRIT_THREADSAFE -DUSE_IPV6
+DEFS=-DWIN32 -D_WINDOWS -DBOOST_THREAD_USE_LIB -DBOOST_SPIRIT_THREADSAFE
 DEBUGFLAGS=-g
 CFLAGS=-mthreads -O2 -w -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameter $(DEBUGFLAGS) $(DEFS) $(INCLUDEPATHS)
 
@@ -36,6 +37,10 @@ ifdef USE_UPNP
  DEFS += -DSTATICLIB -DUSE_UPNP=$(USE_UPNP)
 endif
 
+ifneq (${USE_IPV6}, -)
+	DEFS += -DUSE_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
 
 # TODO: make the mingw builds smarter about dependencies, like the linux/osx builds are