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

src/makefile.osx

@@ -20,6 +20,7 @@ LIBPATHS= \
  -L"$(DEPSDIR)/lib/db48"
 
 USE_UPNP:=1
+USE_IPV6:=1
 
 LIBS= -dead_strip
 
@@ -53,7 +54,7 @@ LIBS += \
 TESTDEFS += -DBOOST_TEST_DYN_LINK
 endif
 
-DEFS=-DMAC_OSX -DMSG_NOSIGNAL=0 -DBOOST_SPIRIT_THREADSAFE -DUSE_IPV6
+DEFS=-DMAC_OSX -DMSG_NOSIGNAL=0 -DBOOST_SPIRIT_THREADSAFE
 
 ifdef RELEASE
 # Compile for maximum compatibility and smallest size.
@@ -105,6 +106,10 @@ else
 endif
 endif
 
+ifneq (${USE_IPV6}, -)
+	DEFS += -DUSE_IPV6
+endif
+
 all: bitcoind
 
 # auto-generated dependencies: