[CMake] Add error checking to catch option manipulation after WEBKIT_OPTION_END

[CMake] Add error checking to catch option manipulation after WEBKIT_OPTION_END
https://bugs.webkit.org/show_bug.cgi?id=152611

Reviewed by Martin Robinson.

If WEBKIT_OPTION_DEFINE is called after WEBKIT_OPTION_END, the option never makes its way
into FEATURE_DEFINES and so will be inconsistently set (available to most of the build, but
not to the DOM bindings generator, for example).

If WEBKIT_OPTION_DEFAULT_PORT_VALUE, WEBKIT_OPTION_CONFLICT, or WEBKIT_OPTION_DEPEND are
called after WEBKIT_OPTION_END, they don't do anything.

Add error checking to catch these bugs.

* Source/cmake/WebKitFeatures.cmake:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@194454 268f45cc-cd09-0410-ab3c-d52691b4dbfc