Refactoring in commit v3.1.0-rc1~688^2~2 (cmTarget: Compute languages
from object libraries on demand, 2014-03-18) taught cmTarget::GetLanguages
to (correctly) include the languages of object library sources.  Previously
this was done only in cmTarget::ComputeLinkImplementationLanguages to
choose the linker language.

The Ninja generator writes out generic build rules for each language
compiled within a target using the rule variables defined in the
directory of the target.  This only needs to be done for languages
actually compiled within the current target.  Switch from using the
cmTarget::GetLanguages method to get the list of languages over to
using cmTarget::GetSourceFiles directly so we do not get the languages
in object libraries.

Strictly speaking this should make no difference because it is not safe
to use objects from a language not enabled in the directory containing
a target or else the link information for the language may not be
considered.  However, in cases when no link information happens to be
needed for a language it was possible in CMake 3.0 and below to enable
a language only in a subdirectory providing an object library, and then
use the objects from a containing directory.  The above change teaches
the Ninja generator to continue working in this case.