gazebo ignition cmake -lpthreads not found

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Building ignition from source on ubuntu 20.04 raises the following error in CMakeError.log

Determining if the function pthread_create exists in the pthreads failed with the following output: Change Dir: /home/thor/gsoc/ign-rendering/build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_6d637/fast && /usr/bin/make -f CMakeFiles/cmTC_6d637.dir/build.make CMakeFiles/cmTC_6d637.dir/build make[1]: Entering directory '/home/thor/gsoc/ign-rendering/build/CMakeFiles/CMakeTmp' Building C object CMakeFiles/cmTC_6d637.dir/CheckFunctionExists.c.o /usr/bin/cc -fPIC -DCHECK_FUNCTION_EXISTS=pthread_create -o CMakeFiles/cmTC_6d637.dir/CheckFunctionExists.c.o -c /usr/share/cmake-3.16/Modules/CheckFunctionExists.c Linking C executable cmTC_6d637 /usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_6d637.dir/link.txt --verbose=1 /usr/bin/cc -fPIC -DCHECK_FUNCTION_EXISTS=pthread_create CMakeFiles/cmTC_6d637.dir/CheckFunctionExists.c.o -o cmTC_6d637 -lpthreads /usr/bin/ld: cannot find -lpthreads collect2: error: ld returned 1 exit status make[1]: * [CMakeFiles/cmTC_6d637.dir/build.make:87: cmTC_6d637] Error 1 make[1]: Leaving directory '/home/thor/gsoc/ign-rendering/build/CMakeFiles/CMakeTmp' make: * [Makefile:121: cmTC_6d637/fast] Error 2