Cannot override CMake options when fetching the library via FetchContent_Declare()

[TheStormN]

CMake 3.13 added support for adjusting option() variables of libraries which are fetched by FetchContent_Declare(). However, the date library CMakeLits.txt file does not support that and so an ugly hacks are needed to make it work. The fix is really trivial and I will submit a PR about that.