clarify why cmake is looking for Java and JNI

storage/connect/CMakeLists.txt

@@ -275,9 +275,11 @@ OPTION(CONNECT_WITH_JDBC "Compile CONNECT storage engine with JDBC support" ON)
 
 IF(CONNECT_WITH_JDBC)
   FIND_PACKAGE(Java 1.6)
-  SET_PACKAGE_PROPERTIES(Java PROPERTIES TYPE OPTIONAL)
+  SET_PACKAGE_PROPERTIES(Java PROPERTIES TYPE OPTIONAL
+    PURPOSE "Required for the CONNECT_JDBC feature")
   FIND_PACKAGE(JNI)
-  SET_PACKAGE_PROPERTIES(JNI PROPERTIES TYPE OPTIONAL)
+  SET_PACKAGE_PROPERTIES(JNI PROPERTIES TYPE OPTIONAL
+    PURPOSE "Required for the CONNECT_JDBC feature")
   IF (JAVA_FOUND AND JNI_FOUND)
     INCLUDE(UseJava)
     INCLUDE_DIRECTORIES(${JAVA_INCLUDE_PATH})