Review libc dependencies and remove when possible

Just for clarification, no plans to remove libc dependency, just did some code analysis to see how much raylib depend on stardard C library. My conclusions:

 - stdlib.h: primary dependency is for malloc() and free()
 - stdio.h: primary dependency is for FILE access, maybe it could go through a custom ABI?
 - string.h: just around 8 functions required
 - math.h: just around 8 functions required
 - others: 1-2 functions required for some other headers

src/textures.c

@@ -65,8 +65,8 @@
 #endif
 
 #include <stdlib.h>             // Required for: malloc(), free()
-#include <string.h>             // Required for: strlen()
 #include <stdio.h>              // Required for: FILE, fopen(), fclose(), fread()
+#include <string.h>             // Required for: strlen() [Used in ImageTextEx()]
 
 #include "utils.h"              // Required for: fopen() Android mapping