Use Official FindIconv (3.11+) to get check the existence of iconv, and add it in dependencies if used

[Gary Gai]

Since CMake 3.11,`FindIconv` is available for searching libcionv.
https://cmake.org/cmake/help/latest/module/FindIconv.html
(Note that the cmake version of xerces-c is 3.12 currently)

There are 2 benefits to replace `HAVE_ICONV_*` by `find_package(Iconv)` + 
`Iconv_IS_BUILT_IN`/`Iconv_FOUND`.


  1.  It’s simpler.
  2.  Users are more convenient to link their custom libiconv provided by 
custom module/config script.
  3.  `target_link_libraries(xerces-c PRIVATE Iconv::Iconv)` is help for cmake 
to analyse dependencies.

My patch:
https://github.com/zjyhjqs/xerces-c/commit/fc8e21bab3b98704d639d0ae2e7df8e347c6e4d5

FindIconv.diff
Description: FindIconv.diff

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