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 $@ $<