FYI: build broken with LLVM 10 since 25 oct 2019

[Johan Engelen]

Before you spend time on it, see:
https://lists.llvm.org/pipermail/llvm-dev/2019-October/136380.html

We can fix this on our end too [1], but I'm waiting for what LLVM 
will do.

-Johan

[1] For example:
if(NOT (LDC_LLVM_VER LESS 1000))
     set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to 
conform to")
     set(CMAKE_CXX_STANDARD_REQUIRED YES)
     set(CMAKE_CXX_EXTENSIONS NO)
endif()