Determining if the pthread_create exist failed with the following output:
Change Dir: /home/timofonic/.cache/pacaur/mysql-workbench-git/src/mysql-workbench/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_94ec8/fast"
/usr/bin/make -f CMakeFiles/cmTC_94ec8.dir/build.make CMakeFiles/cmTC_94ec8.dir/build
make[1]: Entering directory '/home/timofonic/.cache/pacaur/mysql-workbench-git/src/mysql-workbench/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_94ec8.dir/CheckSymbolExists.c.o
/usr/lib/ccache/bin/cc   -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong -fno-plt    -o CMakeFiles/cmTC_94ec8.dir/CheckSymbolExists.c.o   -c /home/timofonic/.cache/pacaur/mysql-workbench-git/src/mysql-workbench/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_94ec8
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_94ec8.dir/link.txt --verbose=1
/usr/lib/ccache/bin/cc -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong -fno-plt   -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now  -rdynamic CMakeFiles/cmTC_94ec8.dir/CheckSymbolExists.c.o  -o cmTC_94ec8 
CMakeFiles/cmTC_94ec8.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text.startup+0x3): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
make[1]: *** [CMakeFiles/cmTC_94ec8.dir/build.make:98: cmTC_94ec8] Error 1
make[1]: Leaving directory '/home/timofonic/.cache/pacaur/mysql-workbench-git/src/mysql-workbench/CMakeFiles/CMakeTmp'
make: *** [Makefile:126: cmTC_94ec8/fast] Error 2

File /home/timofonic/.cache/pacaur/mysql-workbench-git/src/mysql-workbench/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: /home/timofonic/.cache/pacaur/mysql-workbench-git/src/mysql-workbench/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_28cf3/fast"
/usr/bin/make -f CMakeFiles/cmTC_28cf3.dir/build.make CMakeFiles/cmTC_28cf3.dir/build
make[1]: Entering directory '/home/timofonic/.cache/pacaur/mysql-workbench-git/src/mysql-workbench/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_28cf3.dir/CheckFunctionExists.c.o
/usr/lib/ccache/bin/cc   -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong -fno-plt -DCHECK_FUNCTION_EXISTS=pthread_create   -o CMakeFiles/cmTC_28cf3.dir/CheckFunctionExists.c.o   -c /usr/share/cmake-3.9/Modules/CheckFunctionExists.c
Linking C executable cmTC_28cf3
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_28cf3.dir/link.txt --verbose=1
/usr/lib/ccache/bin/cc -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong -fno-plt -DCHECK_FUNCTION_EXISTS=pthread_create  -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now  -rdynamic CMakeFiles/cmTC_28cf3.dir/CheckFunctionExists.c.o  -o cmTC_28cf3 -lpthreads 
/usr/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
make[1]: *** [CMakeFiles/cmTC_28cf3.dir/build.make:98: cmTC_28cf3] Error 1
make[1]: Leaving directory '/home/timofonic/.cache/pacaur/mysql-workbench-git/src/mysql-workbench/CMakeFiles/CMakeTmp'
make: *** [Makefile:126: cmTC_28cf3/fast] Error 2