target latest clang supported standard if -std not given

[K-ballo]

details at #1045 (comment)

other (simpler) options:

• extracting from clang/Basic/LangStandards.def during configuration,
• static_assert on LLVM_VERSION_...,

[K-ballo]

The default flag should enable GNU extensions, since this is what clang would otherwise do.

[mizvekov]

Fixed in #1069