build: don't include winsock2.h on cygwin
build: don't include winsock2.h on cygwin

Under cygwin, winsock2.h is intentionally incompatible with,
<sys/socket.h>, and checking for existence is wrong.

Under mingw, HAVE_WINSOCK2_H is defined on our behalf by
gnulib, in a way that does not interfere with cygwin.

* configure.ac: Drop unnecessary header check.
Reported by Matthias Bolte.

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