Determining if the pthread_create exist failed with the following output:
Change Dir: /home/pi/Slic3r/build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_2aa4b/fast"
/usr/bin/make -f CMakeFiles/cmTC_2aa4b.dir/build.make CMakeFiles/cmTC_2aa4b.dir/build
make[1]: Entering directory '/home/pi/Slic3r/build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_2aa4b.dir/CheckSymbolExists.c.o
/usr/bin/cc    -fPIE   -o CMakeFiles/cmTC_2aa4b.dir/CheckSymbolExists.c.o   -c /home/pi/Slic3r/build/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_2aa4b
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_2aa4b.dir/link.txt --verbose=1
/usr/bin/cc       CMakeFiles/cmTC_2aa4b.dir/CheckSymbolExists.c.o  -o cmTC_2aa4b -rdynamic 
CMakeFiles/cmTC_2aa4b.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text+0x48): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_2aa4b.dir/build.make:97: recipe for target 'cmTC_2aa4b' failed
make[1]: *** [cmTC_2aa4b] Error 1
make[1]: Leaving directory '/home/pi/Slic3r/build/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_2aa4b/fast' failed
make: *** [cmTC_2aa4b/fast] Error 2

File /home/pi/Slic3r/build/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
}

Performing C SOURCE FILE Test HAVE_THREAD_LOCAL_STORAGE failed with the following output:
Change Dir: /home/pi/Slic3r/build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_954dd/fast"
/usr/bin/make -f CMakeFiles/cmTC_954dd.dir/build.make CMakeFiles/cmTC_954dd.dir/build
make[1]: Entering directory '/home/pi/Slic3r/build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_954dd.dir/src.c.o
/usr/bin/cc    -DHAVE_THREAD_LOCAL_STORAGE -fPIE   -o CMakeFiles/cmTC_954dd.dir/src.c.o   -c /home/pi/Slic3r/build/CMakeFiles/CMakeTmp/src.c
/home/pi/Slic3r/build/CMakeFiles/CMakeTmp/src.c:2:5: warning: return type defaults to ‘int’ [-Wimplicit-int]
     __declspec(thread) int tls;
     ^~~~~~~~~~
/home/pi/Slic3r/build/CMakeFiles/CMakeTmp/src.c: In function ‘__declspec’:
/home/pi/Slic3r/build/CMakeFiles/CMakeTmp/src.c:4:20: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘{’ token
     int main(void) {
                    ^
/home/pi/Slic3r/build/CMakeFiles/CMakeTmp/src.c:2:5: warning: type of ‘thread’ defaults to ‘int’ [-Wimplicit-int]
     __declspec(thread) int tls;
     ^~~~~~~~~~
/home/pi/Slic3r/build/CMakeFiles/CMakeTmp/src.c:2:28: error: declaration for parameter ‘tls’ but no such parameter
     __declspec(thread) int tls;
                            ^~~
/home/pi/Slic3r/build/CMakeFiles/CMakeTmp/src.c:6:5: error: expected ‘{’ at end of input
     }
     ^
CMakeFiles/cmTC_954dd.dir/build.make:65: recipe for target 'CMakeFiles/cmTC_954dd.dir/src.c.o' failed
make[1]: *** [CMakeFiles/cmTC_954dd.dir/src.c.o] Error 1
make[1]: Leaving directory '/home/pi/Slic3r/build/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_954dd/fast' failed
make: *** [cmTC_954dd/fast] Error 2

Source file was:

    __declspec(thread) int tls;

    int main(void) {
        return 0;
    }