configure.ac: link with -liconv for locale_charset()

configure.ac: link with -liconv for locale_charset()

On e.g. FreeBSD 10.x, the following situation is common:
- there's iconv implementation in libc, which has no locale_charset()
  function
- there's GNU libiconv installed from Ports Collection

Git build process
- detects that iconv is in libc and thus -liconv is not needed for it
- detects locale_charset in -liconv, but for some reason doesn't add it
  to CHARSET_LIB (as it would do with -lcharset if locale_charset() was
  found there instead of -liconv)
- git doesn't build due to unresolved external locale_charset()

Fix this by adding -liconv to CHARSET_LIB if locale_charset() is
detected in this library.

Signed-off-by: Dmitry Marakasov <amdmi3@amdmi3.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>