Allow use of mingw native on Windows 95 OS.

	Allow use of mingw native on Windows 95 OS.
	* src/gdb/ser-mingw.c (CancelIo): New macro for dynamically loaded
	DLL entry.
	(ser_windows_close): Only call CancelIo if function exists.
	(_initialize_ser_windows): Use LoadLirary/GetProcAddress
	to check for existence of CancelIo function in kernel32 DLL.