[Ada] Fix --enable-build-with-cxx build

[Eric Botcazou]

> Certainly looks better to me.

Thanks, applied.  In any case, it's only a quantitative issue: since gigi will 
very likely use C++ features at some point, it needs to be compiled with the 
C++ compiler, which means that all the FE headers must be extern "C".  So half 
of the 33 files must be modified and I think that it's simpler to modify them 
all and use a single compilation scheme than maintaining two such schemes.

-- 
Eric Botcazou