fix build errors; remove dumb non-working printf annotations; move C++ user-defined-literal thing into the header

tests/cpp_build_test.cpp

@@ -1,12 +1,6 @@
 #include "md.h"
 #include "md.c"
 
-MD_String8 operator "" _md(const char *s, size_t size)
-{
-    MD_String8 str = MD_S8((MD_u8 *)s, (MD_u64)size);
-    return str;
-}
-
 int main(void)
 {
     MD_String8 str = "foobar"_md;