* Ensure we don't need benchmark installed to pass c++ feature checks
Requires removal of some dependencies on benchmark.h from internal
low-level headers, which is a good thing.
Also added better logging to the feature check cmake module.
* attempt to fix sanitizer builds by moving away from llvm head
* extra verbosity
* try clang 13 and add extra logging
* get latest clang and try again
Fixes#974. The `cxx_feature_check` now has an additional
optional argument which can be used to supply extra cmake flags
to pass to the `try_compile` command. The `CMAKE_CXX_STANDARD=14`
flag was determined to be the minimum flag necessary to correctly
compile and run the regex feature checks when compiling with Clang
under Windows (n.b. this does *not* refer to clang-cl, the frontend
to the MSVC compiler). The additional flag is not enabled for any
other compiler/platform tuple.
When cross-compiling, this variable was not set on the second run
of CMake, resulting in the next check failing even though it shouldn't
be run in the first place. Fix this by caching the variable.
