Performing C SOURCE FILE Test CMAKE_HAVE_LIBC_PTHREAD failed with the following output:
Change Dir: /var/tmp/portage/dev-ros/imu_filter_madgwick-1.2.1/work/imu_filter_madgwick-1.2.1_build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/gmake cmTC_c29b9/fast && /usr/bin/gmake -f CMakeFiles/cmTC_c29b9.dir/build.make CMakeFiles/cmTC_c29b9.dir/build
gmake[1]: Entering directory '/var/tmp/portage/dev-ros/imu_filter_madgwick-1.2.1/work/imu_filter_madgwick-1.2.1_build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_c29b9.dir/src.c.o
/usr/bin/x86_64-pc-linux-gnu-gcc    -DNDEBUG -O2 -pipe -march=native -DCMAKE_HAVE_LIBC_PTHREAD   -o CMakeFiles/cmTC_c29b9.dir/src.c.o -c /var/tmp/portage/dev-ros/imu_filter_madgwick-1.2.1/work/imu_filter_madgwick-1.2.1_build/CMakeFiles/CMakeTmp/src.c
Linking C executable cmTC_c29b9
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_c29b9.dir/link.txt --verbose=1
/usr/bin/x86_64-pc-linux-gnu-gcc -O2 -pipe -march=native -DCMAKE_HAVE_LIBC_PTHREAD  -Wl,-O1 -Wl,--as-needed  -rdynamic CMakeFiles/cmTC_c29b9.dir/src.c.o  -o cmTC_c29b9 
/usr/lib/gcc/x86_64-pc-linux-gnu/9.1.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/cmTC_c29b9.dir/src.c.o: in function `main':
src.c:(.text.startup+0x23): undefined reference to `pthread_create'
/usr/lib/gcc/x86_64-pc-linux-gnu/9.1.0/../../../../x86_64-pc-linux-gnu/bin/ld: src.c:(.text.startup+0x2c): undefined reference to `pthread_detach'
/usr/lib/gcc/x86_64-pc-linux-gnu/9.1.0/../../../../x86_64-pc-linux-gnu/bin/ld: src.c:(.text.startup+0x37): undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status
gmake[1]: *** [CMakeFiles/cmTC_c29b9.dir/build.make:87: cmTC_c29b9] Error 1
gmake[1]: Leaving directory '/var/tmp/portage/dev-ros/imu_filter_madgwick-1.2.1/work/imu_filter_madgwick-1.2.1_build/CMakeFiles/CMakeTmp'
gmake: *** [Makefile:121: cmTC_c29b9/fast] Error 2


Source file was:
#include <pthread.h>

void* test_func(void* data)
{
  return data;
}

int main(void)
{
  pthread_t thread;
  pthread_create(&thread, NULL, test_func, NULL);
  pthread_detach(thread);
  pthread_join(thread, NULL);
  pthread_atfork(NULL, NULL, NULL);
  pthread_exit(NULL);

  return 0;
}

Determining if the function pthread_create exists in the pthreads failed with the following output:
Change Dir: /var/tmp/portage/dev-ros/imu_filter_madgwick-1.2.1/work/imu_filter_madgwick-1.2.1_build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/gmake cmTC_a5ec0/fast && /usr/bin/gmake -f CMakeFiles/cmTC_a5ec0.dir/build.make CMakeFiles/cmTC_a5ec0.dir/build
gmake[1]: Entering directory '/var/tmp/portage/dev-ros/imu_filter_madgwick-1.2.1/work/imu_filter_madgwick-1.2.1_build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_a5ec0.dir/CheckFunctionExists.c.o
/usr/bin/x86_64-pc-linux-gnu-gcc    -DNDEBUG -O2 -pipe -march=native -DCHECK_FUNCTION_EXISTS=pthread_create   -o CMakeFiles/cmTC_a5ec0.dir/CheckFunctionExists.c.o -c /usr/share/cmake/Modules/CheckFunctionExists.c
Linking C executable cmTC_a5ec0
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_a5ec0.dir/link.txt --verbose=1
/usr/bin/x86_64-pc-linux-gnu-gcc -O2 -pipe -march=native -DCHECK_FUNCTION_EXISTS=pthread_create  -Wl,-O1 -Wl,--as-needed  -rdynamic CMakeFiles/cmTC_a5ec0.dir/CheckFunctionExists.c.o  -o cmTC_a5ec0 -lpthreads 
/usr/lib/gcc/x86_64-pc-linux-gnu/9.1.0/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
gmake[1]: *** [CMakeFiles/cmTC_a5ec0.dir/build.make:87: cmTC_a5ec0] Error 1
gmake[1]: Leaving directory '/var/tmp/portage/dev-ros/imu_filter_madgwick-1.2.1/work/imu_filter_madgwick-1.2.1_build/CMakeFiles/CMakeTmp'
gmake: *** [Makefile:121: cmTC_a5ec0/fast] Error 2