Have `make ssl-opt` generate `tls13-compat.sh`

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index b1af566..f05e1e3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -403,6 +403,9 @@
 endif()
 
 if(ENABLE_PROGRAMS)
+    set(ssl_opt_target "${MBEDTLS_TARGET_PREFIX}ssl-opt")
+    add_custom_target(${ssl_opt_target})
+
     add_subdirectory(programs)
 endif()