[Bug libstdc++/93602] Missing reference to libiconv in 9.x libstdc++

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93602

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Ah, something did change: we now use automake-1.15.1 and autoconf-2.69 and
that's where all the iconv handling comes from (via the AM_ICONV macro).
Presumably that changed in upstream automake.