test: Do not test unicode if boost::spirit >= 1.72

test: Do not test unicode if boost::spirit >= 1.72

Testing with Boost 1.72 unittest_confutil crashes with:
```
[ RUN      ] ConfUtils.ReadFiles2
Assertion failed: (strict_ischar(ch)), function isblank, file /usr/local/include/boost/spirit/home/support/char_encoding/standard.hpp, line 129.
Abort
```

Which is due to the fact that in Boost 1.72 most of the character
testing has been garded with an assert to make sure char holds an
ascii char.
Like:
```
        static bool
        isblank BOOST_PREVENT_MACRO_SUBSTITUTION (int ch)
        {
            BOOST_ASSERT(strict_ischar(ch));
            return (ch == ' ' || ch == '\t');
        }
```
And ReadFile2 explicitly tests for strings with char value > 0x7f.
So that is certainly going to crash.

Now I can imagine that we would like to be able to have users use
different encodings for things like logfiles.
But for now that is not going to work with the boost:spirit parser

Fixes: https://tracker.ceph.com/issues/43406
Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>