Bootstrap with -Wmissing-prototypes doesn't work for C++

[H.J. Lu]

Since -Wmissing-prototypes doesn't work for C++, using
C++ to bootstrap GCC makes -Wmissing-prototypes useless.
You will see the -Wmissing-prototypes message in stage 1,
but you won't see it in stage3 2/3.

-- 
H.J.