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

src/makefile.linux-mingw

@@ -5,6 +5,7 @@
 DEPSDIR:=/usr/i586-mingw32msvc
 
 USE_UPNP:=0
+USE_IPV6:=1
 
 INCLUDEPATHS= \
  -I"$(CURDIR)" \
@@ -28,7 +29,7 @@ LIBS= \
  -l ssl \
  -l crypto
 
-DEFS=-D_MT -DWIN32 -D_WINDOWS -DBOOST_THREAD_USE_LIB -DBOOST_SPIRIT_THREADSAFE -DUSE_IPV6
+DEFS=-D_MT -DWIN32 -D_WINDOWS -DBOOST_THREAD_USE_LIB -DBOOST_SPIRIT_THREADSAFE
 DEBUGFLAGS=-g
 CFLAGS=-O2 -w -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameter $(DEBUGFLAGS) $(DEFS) $(INCLUDEPATHS)
 
@@ -40,6 +41,10 @@ ifdef USE_UPNP
 	DEFS += -DSTATICLIB -DUSE_UPNP=$(USE_UPNP)
 endif
 
+ifneq (${USE_IPV6}, -)
+	DEFS += -DUSE_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
 
 # TODO: make the mingw builds smarter about dependencies, like the linux/osx builds are