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

Previously this was not visible after building.
diff --git a/Makefile b/Makefile
index ff2084d..dd85e51 100644
--- a/Makefile
+++ b/Makefile
@@ -919,6 +919,7 @@
 	      -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 $@ $<
 
diff --git a/src/haproxy.c b/src/haproxy.c
index cdc11a4..019cc57 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -586,6 +586,9 @@
 #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