In commit 74de9a73

 (cmGlobalGenerator: Delegate storage of enabled
languages to cmState, 2015-04-11) the original LanguageEnabled
member of cmGlobalGenerator was left behind by mistake.  One use
of it in EnableLanguagesFromGenerator (for try_compile) was left,
but the member is not populated anymore.  Drop the member and
teach EnableLanguagesFromGenerator to copy the list of enabled
languages from one cmState to the other.

Reported-by: Matt McCormick <matt.mccormick@kitware.com>