Prefer libiconv for iconv (#44335)

`glibc` and `musl` provide a basic implementation of `iconv` (`iconv`,
`iconv_open`, `iconv_close`), but in practice the installation may be
missing the character encoding methods to make them usable. On Fedora
for example, users need to

```yum install glibc-gconv-extra```

to get the character encodings that `gettext` requires during configure,
namely EUC-JP. Users may not have permissions to install the missing
parts of glibc.

Since Spack can install `libiconv`, it is simpler to use that by
default.

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