cmake: split BUILD_GUI_DEPS option into two

The split is to make this software more packageable. 'make install'
is used by the package building scripts, and should not be installing
vendored dependencies onto the system.

external/unbound/CMakeLists.txt

@@ -230,7 +230,7 @@ if (MINGW)
 endif ()
 
 
-if (BUILD_GUI_DEPS)
+if (INSTALL_VENDORED_LIBUNBOUND)
     install(TARGETS unbound
         ARCHIVE DESTINATION lib)
 endif()