Makefile: add COMPAT_V052 define.

This lets us clearly mark transition features, in a way that they can
be removed after 0.6 is released.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Makefile

@@ -37,6 +37,11 @@ PIE_CFLAGS=-fPIE -fPIC
 PIE_LDFLAGS=-pie
 endif
 
+ifneq ($(NO_COMPAT),1)
+# We support compatibility with pre-0.6.
+COMPAT_CFLAGS=-DCOMPAT_V052=1
+endif
+
 PYTEST := $(shell command -v pytest 2> /dev/null)
 PYTEST_OPTS := -v -x
 ifeq ($(TRAVIS),true)
@@ -161,7 +166,7 @@ ALL_PROGRAMS =
 CPPFLAGS = -DBINTOPKGLIBEXECDIR='"'$(shell sh tools/rel.sh $(bindir) $(pkglibexecdir))'"'
 CWARNFLAGS := -Werror -Wall -Wundef -Wmissing-prototypes -Wmissing-declarations -Wstrict-prototypes -Wold-style-definition
 CDEBUGFLAGS := -std=gnu11 -g -fstack-protector
-CFLAGS = $(CPPFLAGS) $(CWARNFLAGS) $(CDEBUGFLAGS) -I $(CCANDIR) $(EXTERNAL_INCLUDE_FLAGS) -I . -I/usr/local/include $(FEATURES) $(COVFLAGS) $(DEV_CFLAGS) -DSHACHAIN_BITS=48 -DJSMN_PARENT_LINKS $(PIE_CFLAGS)
+CFLAGS = $(CPPFLAGS) $(CWARNFLAGS) $(CDEBUGFLAGS) -I $(CCANDIR) $(EXTERNAL_INCLUDE_FLAGS) -I . -I/usr/local/include $(FEATURES) $(COVFLAGS) $(DEV_CFLAGS) -DSHACHAIN_BITS=48 -DJSMN_PARENT_LINKS $(PIE_CFLAGS) $(COMPAT_CFLAGS)
 
 # We can get configurator to run a different compile cmd to cross-configure.
 CONFIGURATOR_CC := $(CC)