BUILD: Show the value of DEBUG= in haproxy -vv

Previously this was not visible after building.

Makefile

@@ -919,6 +919,7 @@ src/haproxy.o:	src/haproxy.c $(DEP)
 	      -DBUILD_CC='"$(strip $(CC))"' \
 	      -DBUILD_CFLAGS='"$(strip $(VERBOSE_CFLAGS))"' \
 	      -DBUILD_OPTIONS='"$(strip $(BUILD_OPTIONS))"' \
+	      -DBUILD_DEBUG='"$(strip $(DEBUG))"' \
 	      -DBUILD_FEATURES='"$(strip $(BUILD_FEATURES))"' \
 	       -c -o $@ $<
 

src/haproxy.c

@@ -586,6 +586,9 @@ static void display_build_opts()
 #ifdef BUILD_OPTIONS
 	       "\n  OPTIONS = " BUILD_OPTIONS
 #endif
+#ifdef BUILD_DEBUG
+	       "\n  DEBUG   = " BUILD_DEBUG
+#endif
 #ifdef BUILD_FEATURES
 	       "\n\nFeature list : " BUILD_FEATURES
 #endif