fixed issues with building translation via cmake

commit 3effecd added an "if" block in CMakeLists with a variable that
does not exist ("NLS"). This breaks running translation updates for
every cmake user. Changed the variable to the existing "ENABLE_NLS".

@Pentarctagon I don't know if the travis script has to be adjusted as
well. Please test.

(cherry-picked from commit 64fe3988d8)

CMakeLists.txt

@@ -547,11 +547,11 @@ if(ENABLE_POT_UPDATE_TARGET)
 endif(ENABLE_POT_UPDATE_TARGET)
 
 # get languages
-if(NLS)
+if(ENABLE_NLS)
 	file(READ po/LINGUAS LINGUAS)
 	string(REPLACE "\n" "" LINGUAS ${LINGUAS})
 	separate_arguments(LINGUAS)
-endif(NLS)
+endif(ENABLE_NLS)
 
 #
 # Include subdirectories