https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122744
--- Comment #2 from Sam James <sjames at gcc dot gnu.org> --- You have to make sure CC/CXX points to GCC from that GNAT community edition too as it expects the driver to know how to build Ada. I have some local hacks for making this work with arbitrary paths to some Ada compiler stashed away somewhere but it's not upstream and not in the form of proper patches.
