Mark test/unit_test.c as obsolete.

Since the new CMake based build system will use other files for
unit tests (public_func.c, public_server.c, private.c, private_exe.c
and others), this file is no longer maintained, and will be removed
in the future.

See also #269

test/unit_test.c

@@ -20,6 +20,16 @@
  * THE SOFTWARE.
  */
 
+/* Note: The unit_test.c file is mostly obsolete, since the current unit
+ * tests are performed by the CMake build framework based on the
+ * CMakeList (https://github.com/civetweb/civetweb/blob/master/test/CMakeLists.txt).
+ * The tests do no longer use unit_test.c but public_func.c, public_server.c,
+ * private.c, private_exe.c and others.
+ * This file is left here for reference and will be removed in the future.
+ * It is no longer actively maintained.
+ */
+
+
 /* Unit test for the civetweb web server. Tests embedded API.
  */
 #define USE_WEBSOCKET