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>