U651 - avoid declaring errno unnecessarily

Some changes for standard C platforms, to avoid declaring errno explictly.
Such platforms should declare errno in errno.h, which is already included
in the files in question.

src/files.c

@@ -20,6 +20,10 @@
 # if (_MSC_VER >= 600)
 #  define SKIP_ERRNO
 # endif
+#else
+# ifdef NHSTDC
+#  define SKIP_ERRNO
+# endif
 #endif
 #ifndef SKIP_ERRNO
 # ifdef _DCC