Revived the getenv macro for dln_find.c

This partially reverts commit "Windows: Improve readablity of
getenv() encoding" 14453a256d.

The `getenv` macro defined here is to also substitute the function
in dln_find.c, not only in this file.

win32/win32.c

@@ -77,6 +77,7 @@ static char *w32_getenv(const char *name, UINT cp);
 #define DLN_FIND_EXTRA_ARG_DECL ,UINT cp
 #define DLN_FIND_EXTRA_ARG ,cp
 #define rb_w32_stati128(path, st) w32_stati128(path, st, cp, FALSE)
+#define getenv(name) w32_getenv(name, cp) /* Necessarily For dln.c */
 #undef CharNext
 #define CharNext(p) CharNextExA(cp, (p), 0)
 #define dln_find_exe_r rb_w32_udln_find_exe_r