Work around a bug in libcpp that pulls in the optional system iconv
if present instead of relying on the port we actually depend on.
The issue is that /usr/include/iconv.h has #include <stdbool.h> which
in turn, since both are included very late in the game, conflicts with
similar definintions by libcpp itself.
libstdc++ does not seem to require adjustments, so PR 161417 may not
be relevant in full.