squash! mingw: HOT FIX: work around environment issues -- again

cURL actually uses "cp" + GetACP():
https://github.com/bagder/curl/blob/aa5808b5/lib/easy.c#L157-L162

Author: dscho

compat/mingw.c

@@ -2332,8 +2332,12 @@ void mingw_startup()
 	 * Avoid a segmentation fault when cURL tries to set the CHARSET
 	 * variable and putenv() barfs at our nedmalloc'ed environment.
 	 */
-	if (!getenv("CHARSET"))
-		setenv("CHARSET", "cp1252", 1);
+	if (!getenv("CHARSET")) {
+		struct strbuf buf = STRBUF_INIT;
+		strbuf_addf(&buf, "cp%u", GetACP());
+		setenv("CHARSET", buf.buf, 1);
+		strbuf_release(&buf);
+	}
 
 	/* initialize critical section for waitpid pinfo_t list */
 	InitializeCriticalSection(&pinfo_cs);