Cross-compilation errors with std::async

Question

Background

1. I built a toolchain for my ARMv7 Raspberry Pi using crosstool-ng on my Ubuntu 18.04. Due to a problem with GCC 7.1 compilation of GCC 6.3.0 I did some modifications to ubsan.c file.
2. I built the required libraries on the Raspberry Pi itself, used the symlink utility to change symlinks from absolute to relative, compressed the /usr and /lib directories for transfer and installed them in my working sysroot directory.
3. This toolchain has been shown to work for a number of large projects.

The problem

I have built a simple application using the paho.mqtt.cpp library. This application uses the C++11 standard, as required by the library. The application compiles and executes when compiled on my development machine and also when compiled on the Raspberry Pi. However, when I use the abovementioned toolchain, I get an error related to the library's use of std::async.

In file included from /home/hicklin/CLionProjects/mqttTest/main.cpp:7:0:
/opt/pi/sysroot/usr/local/include/mqtt/client.h: In member function 'virtual void mqtt::client::connected(const string&)':
/opt/pi/sysroot/usr/local/include/mqtt/client.h:71:76: error: invalid use of incomplete type 'class std::future<void>'
   std::async(std::launch::async, &callback::connected, userCallback_, cause);
                                                                            ^
In file included from /opt/pi/sysroot/usr/local/include/mqtt/client.h:28:0,
                 from /home/hicklin/CLionProjects/mqttTest/main.cpp:7:
/opt/pi/x-tools/arm-unknown-linux-gnueabihf/arm-unknown-linux-gnueabihf/include/c++/6.3.1/future:115:11: note: declaration of 'class std::future<void>'
     class future;
           ^~~~~~

Attempted fixes

1. I tried using the -nostdinc flag to stop the compiler from using its standard headers and use the ones in the sysroot. The error persisted.
2. I have tried replacing the future file referred to in the error with the one installed in the Pi (it only has a very minor difference). The error persisted.

My question

Would anyone be able to give an indication to the cause of this error and possible things to look into? Should I try to rebuild the toolchain differently or perhaps I am missing some flags?