Incorrect detection of std::in_place

[Pesa]

optional.hpp seems to rely only on the C++ standard version in use to choose between the std::in_place facility from the standard library or its own implementation.

https://github.com/martinmoene/optional-lite/blob/86db12f3b6c2026ad83efd51c5ce3622bb06f1b8/include/nonstd/optional.hpp#L88-L99

However, simply compiling in C++17 mode doesn't guarantee that the standard library contains these facilities. For instance, this detection technique fails on Ubuntu 16.04 when compiling with clang-5.0 (and the default libstdc++):

$ clang++-5.0 -xc++ -std=c++17 -ferror-limit=5 -c -o /dev/null - <<< '#include "optional.hpp"'
In file included from <stdin>:1:
./optional.hpp:94:12: error: no member named 'in_place' in namespace 'std'
using std::in_place;
      ~~~~~^
./optional.hpp:95:12: error: no member named 'in_place_type' in namespace 'std'
using std::in_place_type;
      ~~~~~^
./optional.hpp:96:12: error: no member named 'in_place_index' in namespace 'std'
using std::in_place_index;
      ~~~~~^
./optional.hpp:97:12: error: no member named 'in_place_t' in namespace 'std'
using std::in_place_t;
      ~~~~~^
./optional.hpp:98:12: error: no member named 'in_place_type_t' in namespace 'std'
using std::in_place_type_t;
      ~~~~~^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
6 errors generated.

[brennanleez-coder]

elseif (UNIX)

Turn on C++11

add_definitions(-std=c++17)
set(CMAKE_CXX_FLAGS_RELEASE "-O3")
elseif (APPLE)

Turn on C++17

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++17")
set(CMAKE_CXX_FLAGS_RELEASE "-O3")
endif (WIN32)

in openpose/CmakeLists make these changes then recompile. Allows me to compile but somehow the detections are not working.