[Bug bootstrap/88623] gcc build uses CXX_FOR_BUILD but files have .c extension

[andrew at ishiboo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88623

--- Comment #4 from Andrew Paprocki <andrew at ishiboo dot com> ---
... relying on the fact that `g++` will compile the `.c` files as C.