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

configure.ac

@@ -890,7 +890,7 @@ GIT_CONF_SUBST([HAVE_STRINGS_H])
 # and libcharset does
 CHARSET_LIB=
 AC_CHECK_LIB([iconv], [locale_charset],
-       [],
+       [CHARSET_LIB=-liconv],
        [AC_CHECK_LIB([charset], [locale_charset],
                      [CHARSET_LIB=-lcharset])])
 GIT_CONF_SUBST([CHARSET_LIB])