[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 #3 from Andrew Paprocki <andrew at ishiboo dot com> ---
Sorry, I said 'libgcc', but I meant the 'gcc/' sub-directory. It is always
using the C++ compiler for the build and relying on the fact that `g++`