Build fails on Gentoo with "undefined reference to symbol 'pthread_create@@GLIBC_2.2.5'"

[slaufmann]

I tried building as described in the Build.me file.
While running make the following error occured:

[ 21%] Linking CXX executable mandelbrot_ff
/usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/mandelbrot_ff.dir/renderthread.cpp.o: undefined reference to symbol 'pthread_create@@GLIBC_2.2.5'
/lib64/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
make[2]: *** [examples/qt-mandelbrot/fastflow/CMakeFiles/mandelbrot_ff.dir/build.make:211: examples/qt-mandelbrot/fastflow/mandelbrot_ff] Error 1
make[1]: *** [CMakeFiles/Makefile2:599: examples/qt-mandelbrot/fastflow/CMakeFiles/mandelbrot_ff.dir/all] Error 2
make: *** [Makefile:139: all] Error 2

My kernel version and system information as of uname -a is:
4.9.16-gentoo #1 SMP PREEMPT Fri Apr 21 13:14:04 CEST 2017 x86_64 Intel(R) Core(TM) i5-6260U CPU @ 1.80GHz GenuineIntel GNU/Linux

A full log of the terminal output can be found on this gist: https://gist.github.com/slaufmann/3a0d1c6209f921264f098b7c4a794308#file-buildloggentoo

If I could help with providing any additional information please let me know.

Thanks.