Various bugs can cause crashes, so don't use that function before ICU
53.  It will fall back to the code path used for other encodings.

Since we now tie the function availability to an ICU version, we don't
need the configure test anymore.  That also resolves the issue that the
test result was previously hardcoded for Windows.

researched by Daniel Verite <daniel@manitou-mail.org>, Peter Geoghegan
<pg@bowt.ie>, Tom Lane <tgl@sss.pgh.pa.us>

Discussion: https://www.postgresql.org/message-id/flat/f1438ec6-22aa-4029-9a3b-26f79d330e72%40manitou-mail.org