Fix openpty detection on openbsd

(cherry picked from commit e255a8ca63)

CMakeLists.txt

@@ -443,6 +443,7 @@ if( HAVE_RES_INIT )
 endif( HAVE_RES_INIT)
 
 set( CMAKE_REQUIRED_LIBRARIES util )
+if( HAVE_PTY_H )
   check_c_source_runs("
       #include <pty.h>
       int main(int argc, char* argv) {
@@ -453,6 +454,18 @@ check_c_source_runs("
     }"
     HAVE_OPENPTY
   )
+elseif( HAVE_UTIL_H )
+  check_c_source_runs("
+      #include <util.h>
+      int main(int argc, char* argv) {
+        int master_fd, slave_fd;
+        int result;
+        result = openpty(&master_fd, &slave_fd, 0, 0, 0);
+        return 0;
+    }"
+    HAVE_OPENPTY
+  )
+endif( )
 if( HAVE_OPENPTY )
   set( LIB_UTIL util )
 endif( )