[libvoikko] --disable-malaga added

[Harri Pitkänen]

I have just added a configure option for disabling the Malaga backend of 
libvoikko. This option may be used if you are only interested in developing 
the alternative backends. Even then I'm not sure if it really is worth using 
it since you will only save a few seconds of compilation time. And if you 
disable the backend you will no longer be able to run most of the tests which 
will make it quite risky to make any changes outside backend specific code.

But you may find it useful if you like to be extra sure that the Malaga is not 
accidentally used or something like that.

Harri