Log Message:
Fail if this package is to be built under Linux.  This is normal
for Linux. Most (all?) Linux distros don't provide a libiconv
package, because GLIBC's iconv support is good enough.

At this time, using PKG_FAIL_REASON instead of PKG_SKIP_REASON
so we can catch examples of packages depending on this.

The builtin.mk was improved to help stop it from being used under
Linux.

This was okayed by Rene Hexel, the maintainer.

Todo: fix abiword build because it should not require this libiconv
package under Linux.