PATCH to pre-select languages to be build

[Joern Rennecke]

> By default we should build all languages.  The user must explicitly specify
> that they do not want particular languages.  I think we've agreed on that.

Well, how about some alternate syntax to do this that doesn't require
to know all the languages?

E.g.:  --disable-default-languages --enable-c --enable-fortran
to build only c and fortran.

No, I'm not volunteering to implement it ;-)