cmake: gcc warning management

* disable unused value warning (don't like the syntax 0&&)
* remove -Wno-ignored-attributes (c option). It replaces gcc warning
    by an unsupported option warning...
cmake: install mo file into Langs/id_code instead of Langs/id_code/LC_MESSAGES

Therefore it uses the same locations as windows.
wx3.0: clang complains about support of reference in varargs
wx3.0: partially revert 1fda2a611c55ab1cae10df548cbd98ffada327d8

I wrongly removed code that set the language directory on Windows.
Linux uses another way (don't know which one)

close issue #189
wx3.0: compilation fix for arch linux

* Manually cast WxGetTranslation
* Accept string as format parameter of pxWindowTextWriter
* Manually convert wxString to wide string

Note: Wx setup.h is not the same between Debian and Arch. Unfortunately it
generated various compilations errors on wx code.

Close issue #172