Since commit c30d06b7 (FindIntl: Add imported target, 2020-10-06,
v3.20.0-rc1~687^2) we use `check_symbol_exists` to check whether the
`intl` library is built in to the C library.  On some platforms the
tested symbols are provided as macros so the check passes without
linking any symbol.  Instead, check whether a sample source file both
compiles and links.

Fixes: #21979