libidn: update to 1.15 and other fixes

Closes #715

* Add target uninstall.
* Remove stray /usr/share/emacs files.
* Remove stray mono (host, autodetected) dll installation in
  $(TARGET_DIR)/usr/lib by disabling C# altogether.
* Disable java.
* Add new option to install idn binary (default N, usually not required).

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>