Fix Clang builds on Windows (#1021)

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.

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