Use Solaris iconv

Currently hundreds of tests FAIL on Solaris with

warning: could not convert '...' from the host encoding (ISO-8859-1) to UTF-32.
This normally should not happen, please file a bug report.

This happens because the system headers don't define __STDC_ISO_10646__
which was only introduced in C17.  However, Solaris libc only conforms
to C11.  Consequently PHONY_ICONV is defined, leading to those failures.

However, Solaris 11.4 iconv is good enough to use: enabling its use
fixes 300+ failures.  While it is likely that this also works with older
Solaris 11 versions, those are unsupported and no longer tested.  In case
of problems, one can always use GNU libiconv.

The results are on par with those using GNU libiconv 1.17, while
forcibly disabling HAVE_ICONV on Linux/x86_64 adds causes the same
amount of failures.

Tested on sparcv9-sun-solaris2.11, x86_64-pc-solaris2.11, and
x86_64-pc-solaris2.11.