make: Add an option to compile with address sanitizer

Currently only works with `gcc` due to google/sanitizers#1028, so
configure makes sure we warn if clang with ASAN is attempted.

According to [my benchmarks][benchmarks] the performance degradation
is small enough to have it active always.

[benchmarks]: https://github.com/ElementsProject/lightning/issues/2277#issuecomment-455897417

Signed-off-by: Christian Decker <decker.christian@gmail.com>

Makefile

@@ -24,6 +24,12 @@ VG=valgrind -q --error-exitcode=7
 VG_TEST_ARGS = --track-origins=yes --leak-check=full --show-reachable=yes --errors-for-leak-kinds=all
 endif
 
+ifneq ($(ASAN),0)
+SANITIZER_FLAGS=-fsanitize=address
+else
+SANITIZER_FLAGS=
+endif
+
 ifeq ($(DEVELOPER),1)
 DEV_CFLAGS=-DCCAN_TAKE_DEBUG=1 -DCCAN_TAL_DEBUG=1
 else
@@ -179,13 +185,13 @@ 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
+CDEBUGFLAGS := -std=gnu11 -g -fstack-protector $(SANITIZER_FLAGS)
 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)
 
-LDFLAGS = $(PIE_LDFLAGS)
+LDFLAGS = $(PIE_LDFLAGS) $(SANITIZER_FLAGS)
 ifeq ($(STATIC),1)
 LDLIBS = -L/usr/local/lib -Wl,-dn -lgmp -lsqlite3 -lz -Wl,-dy -lm -lpthread -ldl $(COVFLAGS)
 else