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

Add version and license to meson.build

#1179 Reordered the code. It seems to stop clang 3.4.2 in RHEL 7 from crash…

* Make the conversion operator SFINAE correct.
* Workaround a GCC bug with some traits in type_traits.hpp

The first bullet-point implies that every `get`/`get_ptr` be SFINAE
correct as well.

Use a version check to provide backwards comatible CMake imported target names

See https://lgtm.com/projects/g/nlohmann/json

See https://blog.travis-ci.com/2018-09-13-xcode-10-is-now-available?utm_source=twitter&utm_medium=web&utm_campaign=xcode10_gm

Better error 305

Move lambda out of unevaluated context

Remove static asserts