Makefile.am: link with libiconv when needed

Fixes issues on systems with libiconv separate from libc (e.g. Darwin/macOS, uclibc).

We already ask configure to do detection for us of whether we need an explicit library link line for libintl and libiconv, so let's use the result.

This will have no effect if the variable is empty of course, as will be the case on many systems (everywhere it worked before).

We've been carrying this patch in Gentoo for quite some time with no issues.

Bug: https://bugs.gentoo.org/366885 Bug: https://bugs.gentoo.org/499940 Signed-off-by: Sam James sam@gentoo.org