Add a function that prepares the language configure option.
It is needed in at least two places, some commonalisation is needed. ;-)
Unfortunately, it is no longer possible to print warnings about experimental
languages any more. Anyway, the experimental status is clearly indicated
in the menuconfig. so it should not be a surprise if the build breaks. :-/
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>