Use multiline comments in C files

So there were actually only two instances of C99-style end-of-line
comments in C files - and technically one of them was just a C file
including MakeAndPlay.h.

desktop_version/src/ThirdPartyDeps.c

@@ -1,6 +1,6 @@
 #include <SDL_stdinc.h>
 
-// Handle third-party dependencies' needs here
+/* Handle third-party dependencies' needs here */
 
 void* lodepng_malloc(size_t size)
 {