configure: test for pwritev

MacOS doesn't have it apparently.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

configure

@@ -292,6 +292,29 @@ code=
 #error "Not modern GCC"
 #endif
 /*END*/
+var=HAVE_PWRITEV
+desc=pwritev() defined
+style=DEFINES_EVERYTHING|EXECUTE|MAY_NOT_COMPILE
+code=
+#include <sys/uio.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+int main(void)
+{
+	struct iovec iov[2];
+	int fd = open("/dev/null", O_WRONLY);
+
+	iov[0].iov_base = "hello";
+	iov[0].iov_len = 5;
+	iov[1].iov_base = " world";
+	iov[1].iov_len = 6;
+	if (pwritev(fd, iov, 2, 0) == 11)
+		return 0;
+	return 1;
+}
+/*END*/
 EOF
 mv $CONFIG_VAR_FILE.$$ $CONFIG_VAR_FILE