Utils: Add our own ASSERT macro

Using `assert` from `<cassert>` leads to unused variable warnings in release builds.
Define the `ASSERT` macro which compiles to a no-op in release builds but still avoids
the warnings.

src/zm_font.h

@@ -20,7 +20,6 @@
 
 #include "zm_define.h"
 #include <array>
-#include <cassert>
 #include "span.hpp"
 #include <string>
 #include <vector>

src/zm_utils.h

@@ -31,6 +31,14 @@
 #include <sys/time.h>
 #include <vector>
 
+
+#ifdef NDEBUG
+#define ASSERT(x) do { (void) sizeof(x); } while (0)
+#else
+#include <cassert>
+#define ASSERT(x) assert(x)
+#endif
+
 typedef std::vector<std::string> StringVector;
 
 std::string Trim(const std::string &str, const std::string &char_set);