update CMakeLists to correctly compile sources - update readme to reflect that

factor out reused functions to shared.cpp/shared.h

shared.h

@@ -0,0 +1,26 @@
+//
+// Created by neo on 5/26/2023.
+//
+
+#ifndef WINDOWSXPKG_SHARED_H
+#define WINDOWSXPKG_SHARED_H
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <string.h>
+
+#include <openssl/bn.h>
+#include <openssl/ec.h>
+#include <openssl/sha.h>
+#include <openssl/rand.h>
+
+extern uint8_t cset[];
+
+void endian(uint8_t *data, int len);
+void unbase24(uint32_t *x, uint8_t *c);
+void base24(uint8_t *c, uint32_t *x);
+void print_product_key(uint8_t *pk);
+void print_product_id(uint32_t *pid);
+
+#endif //WINDOWSXPKG_SHARED_H