This is a follow-up to #1210.

Thirdparty benchmark: Fix Clang detection.

CMake 3.0+ refers to macOS’ Clang as AppleClang, which would fail a STREQUAL check. Fixed by changing it to MATCHES.

Fix warning C4127: conditional expression is constant

docs: Add additional CMake documentation

Turn off additional deprecation warnings for GCC.

In follow up from the conversation in #1269, this adds the
`-Wno-deprecate-declarations` flag to unit tests to allow them to
test deprecated APIs without all the noisy compiler warnings.
This also refactors the setting of build properties for test targets
to use `target_<...>` commands instead of `set_target_properties()`.

Fix broken cmake imported target alias

Fix issue #1237

Add a get overload taking a parameter.

Closes #1222

Closes #1222

unit-testsuites.cpp: fix hangup if file not found

If run from the wrong directory, std::ifstream f("test/data/big-list-of-naughty-strings/blns.json"); will not find the file and thus f.eof() will never return true.
Use canonical C++ file reading loop from https://gehrcke.de/2011/06/reading-files-in-c-using-ifstream-dealing-correctly-with-badbit-failbit-eofbit-and-perror/ instead.

Takes an lvalue reference, and returns the same reference.

This allows non-default constructible types to be converted without
specializing adl_serializer.
This overload does not require CopyConstructible either.

Implements #1227

GCC 48

See https://github.com/nlohmann/json/pull/212 for details