For Debug build, #define macro DEBUG.
diff --git a/md4c/CMakeLists.txt b/md4c/CMakeLists.txt
index 8e24ec4..b18629e 100644
--- a/md4c/CMakeLists.txt
+++ b/md4c/CMakeLists.txt
@@ -1,2 +1,4 @@
+set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -DDEBUG")
+
add_library(md4c STATIC md4c.c md4c.h)