Automatically disable NLS if gettext and/or libiconv are missing. On some platforms...

Automatically disable NLS if gettext and/or libiconv are missing.  On some platforms, such as Windows and OS X, NLS can't be enabled without a lot of custom configuration, so the build simply fails on those platforms unless -DENABLE_NLS=0 is specified.  This change makes the CMake system behave more like autotools used to, and it is consistent with the detection behavior of other non-essential modules, such as GnuTLS.

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4531 3789f03b-4d11-0410-bbf8-ca57d06f2519