[RFA] Fix libsupc++/Makefile.in

[B. Kosnik]

> This seems to fix it, but I'm not 100% certain this is right.
> However, it does appear that LTCOMPILE is intended for C file
> compilations, so having C++ flags is daft.

Yep.

> OK to commit?

This patch is conceptually fine, but what you need to do is edit
libsupc++/Makefile.am and then auto-generate libsupc++/Makefile.in.

(The best way to do that is, at the top level of libstdc++-v3:
aclocal; autoconf; autoheader; automake)

best,
benjamin