Don't unnecessarily redefine 'socklen_t' type in MinGW builds.

Don't unnecessarily redefine 'socklen_t' type in MinGW builds.

The original configure-time tests in gdb/ and gdbserver/ failed to
detect that 'socklen_t' is defined in MinGW headers because the test
program included only sys/socket.h, which is absent in MinGW system
headers.  However on MS-Windows this data type is declared in another
header, ws2tcpip.h.  The modified test programs try using ws2tcpip.h
if sys/socket.h is unavailable.

Thanks to Joel Brobecker who helped me regenerate the configure
scripts and the config.in files.

gdb/ChangeLog:
2020-07-26  Eli Zaretskii  <eliz@gnu.org>

	* configure.ac (AC_CHECK_HEADERS): Check for sys/socket.h and
	ws2tcpip.h.  When checking whether socklen_t type is defined, use
	ws2tcpip.h if it is available and sys/socket.h isn't.
	* configure: Regenerate.
	* config.in: Regenerate.

gdbserver/ChangeLog:
2020-07-26  Eli Zaretskii  <eliz@gnu.org>

	* configure.ac (AC_CHECK_HEADERS): Add ws2tcpip.h.
	When checking whether socklen_t type is defined, use ws2tcpip.h if
	it is available and sys/socket.h isn't.
	* configure: Regenerate.
	* config.in: Regenerate.