Determining if the pthread_create exist failed with the following output:
Change Dir: /var/tmp/portage/app-admin/elektra-0.8.20/work/elektra-0.8.20-abi_x86_64.amd64/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/gmake" "cmTC_f6abf/fast"
/usr/bin/gmake -f CMakeFiles/cmTC_f6abf.dir/build.make CMakeFiles/cmTC_f6abf.dir/build
gmake[1]: Entering directory '/var/tmp/portage/app-admin/elektra-0.8.20/work/elektra-0.8.20-abi_x86_64.amd64/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_f6abf.dir/CheckSymbolExists.c.o
/usr/bin/x86_64-pc-linux-gnu-gcc   -O2 -pipe -march=native -std=gnu99  -Wno-deprecated-declarations  -Wstrict-prototypes  -Wno-long-long -pedantic -Wno-variadic-macros -Wall -Wextra -Wno-overlength-strings -Wsign-compare -Wfloat-equal -Wformat -Wformat-security -Wshadow -Wcomments -Wtrigraphs -Wundef -Wuninitialized -Winit-self -Wmaybe-uninitialized -Wsign-compare -Wfloat-equal    -o CMakeFiles/cmTC_f6abf.dir/CheckSymbolExists.c.o   -c /var/tmp/portage/app-admin/elektra-0.8.20/work/elektra-0.8.20-abi_x86_64.amd64/CMakeFiles/CMakeTmp/CheckSymbolExists.c
/var/tmp/portage/app-admin/elektra-0.8.20/work/elektra-0.8.20-abi_x86_64.amd64/CMakeFiles/CMakeTmp/CheckSymbolExists.c: In function ‘main’:
/var/tmp/portage/app-admin/elektra-0.8.20/work/elektra-0.8.20-abi_x86_64.amd64/CMakeFiles/CMakeTmp/CheckSymbolExists.c:8:11: warning: ISO C forbids conversion of function pointer to object pointer type [-Wpedantic]
   return ((int*)(&pthread_create))[argc];
           ^
Linking C executable cmTC_f6abf
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_f6abf.dir/link.txt --verbose=1
/usr/bin/x86_64-pc-linux-gnu-gcc -O2 -pipe -march=native -std=gnu99  -Wno-deprecated-declarations  -Wstrict-prototypes  -Wno-long-long -pedantic -Wno-variadic-macros -Wall -Wextra -Wno-overlength-strings -Wsign-compare -Wfloat-equal -Wformat -Wformat-security -Wshadow -Wcomments -Wtrigraphs -Wundef -Wuninitialized -Winit-self -Wmaybe-uninitialized -Wsign-compare -Wfloat-equal   -Wl,-O1 -Wl,--as-needed  -rdynamic CMakeFiles/cmTC_f6abf.dir/CheckSymbolExists.c.o  -o cmTC_f6abf 
CMakeFiles/cmTC_f6abf.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text.startup+0x3): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
gmake[1]: *** [CMakeFiles/cmTC_f6abf.dir/build.make:87: cmTC_f6abf] Error 1
gmake[1]: Leaving directory '/var/tmp/portage/app-admin/elektra-0.8.20/work/elektra-0.8.20-abi_x86_64.amd64/CMakeFiles/CMakeTmp'
gmake: *** [Makefile:126: cmTC_f6abf/fast] Error 2

File /var/tmp/portage/app-admin/elektra-0.8.20/work/elektra-0.8.20-abi_x86_64.amd64/CMakeFiles/CMakeTmp/CheckSymbolExists.c:
/* */
#include <pthread.h>

int main(int argc, char** argv)
{
  (void)argv;
#ifndef pthread_create
  return ((int*)(&pthread_create))[argc];
#else
  (void)argc;
  return 0;
#endif
}

Determining if the function pthread_create exists in the pthreads failed with the following output:
Change Dir: /var/tmp/portage/app-admin/elektra-0.8.20/work/elektra-0.8.20-abi_x86_64.amd64/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/gmake" "cmTC_726ef/fast"
/usr/bin/gmake -f CMakeFiles/cmTC_726ef.dir/build.make CMakeFiles/cmTC_726ef.dir/build
gmake[1]: Entering directory '/var/tmp/portage/app-admin/elektra-0.8.20/work/elektra-0.8.20-abi_x86_64.amd64/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_726ef.dir/CheckFunctionExists.c.o
/usr/bin/x86_64-pc-linux-gnu-gcc   -O2 -pipe -march=native -std=gnu99  -Wno-deprecated-declarations  -Wstrict-prototypes  -Wno-long-long -pedantic -Wno-variadic-macros -Wall -Wextra -Wno-overlength-strings -Wsign-compare -Wfloat-equal -Wformat -Wformat-security -Wshadow -Wcomments -Wtrigraphs -Wundef -Wuninitialized -Winit-self -Wmaybe-uninitialized -Wsign-compare -Wfloat-equal -DCHECK_FUNCTION_EXISTS=pthread_create   -o CMakeFiles/cmTC_726ef.dir/CheckFunctionExists.c.o   -c /usr/share/cmake/Modules/CheckFunctionExists.c
Linking C executable cmTC_726ef
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_726ef.dir/link.txt --verbose=1
/usr/bin/x86_64-pc-linux-gnu-gcc -O2 -pipe -march=native -std=gnu99  -Wno-deprecated-declarations  -Wstrict-prototypes  -Wno-long-long -pedantic -Wno-variadic-macros -Wall -Wextra -Wno-overlength-strings -Wsign-compare -Wfloat-equal -Wformat -Wformat-security -Wshadow -Wcomments -Wtrigraphs -Wundef -Wuninitialized -Winit-self -Wmaybe-uninitialized -Wsign-compare -Wfloat-equal -DCHECK_FUNCTION_EXISTS=pthread_create  -Wl,-O1 -Wl,--as-needed  -rdynamic CMakeFiles/cmTC_726ef.dir/CheckFunctionExists.c.o  -o cmTC_726ef -lpthreads 
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
gmake[1]: *** [CMakeFiles/cmTC_726ef.dir/build.make:87: cmTC_726ef] Error 1
gmake[1]: Leaving directory '/var/tmp/portage/app-admin/elektra-0.8.20/work/elektra-0.8.20-abi_x86_64.amd64/CMakeFiles/CMakeTmp'
gmake: *** [Makefile:126: cmTC_726ef/fast] Error 2