Determining if the pthread_create exist failed with the following output:
Change Dir: /var/tmp/portage/dev-db/clickhouse-18.12.17/work/clickhouse-18.12.17_build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/ninja cmTC_872a7 
[1/2] Building C object CMakeFiles/cmTC_872a7.dir/CheckSymbolExists.c.o
[2/2] Linking C executable cmTC_872a7
FAILED: cmTC_872a7 
: && /usr/bin/x86_64-pc-linux-gnu-gcc -O2 -pipe -march=native -fdiagnostics-color=always  -pipe -msse4.1 -msse4.2 -mpopcnt -fno-omit-frame-pointer  -Wall  -Wl,-O1 -Wl,--as-needed   -rdynamic CMakeFiles/cmTC_872a7.dir/CheckSymbolExists.c.o  -o cmTC_872a7   && :
/usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/cmTC_872a7.dir/CheckSymbolExists.c.o: in function `main':
CheckSymbolExists.c:(.text.startup+0x3): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.

File /var/tmp/portage/dev-db/clickhouse-18.12.17/work/clickhouse-18.12.17_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_UNW_INIT_LOCAL2 failed with the following output:
Change Dir: /var/tmp/portage/dev-db/clickhouse-18.12.17/work/clickhouse-18.12.17_build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/ninja cmTC_3d302 
[1/2] Building CXX object CMakeFiles/cmTC_3d302.dir/src.cxx.o
FAILED: CMakeFiles/cmTC_3d302.dir/src.cxx.o 
/usr/bin/x86_64-pc-linux-gnu-g++    -DNDEBUG -O2 -pipe -march=native -fdiagnostics-color=always  -pipe -msse4.1 -msse4.2 -mpopcnt  -fno-omit-frame-pointer  -Wall  -Wnon-virtual-dtor -DHAVE_UNW_INIT_LOCAL2 -fPIE -o CMakeFiles/cmTC_3d302.dir/src.cxx.o -c src.cxx
src.cxx: In function ‘int main()’:
src.cxx:8:43: error: ‘UNW_INIT_SIGNAL_FRAME’ was not declared in this scope
        unw_init_local2(&cursor, &context, UNW_INIT_SIGNAL_FRAME);
                                           ^~~~~~~~~~~~~~~~~~~~~
src.cxx:8:8: error: ‘unw_init_local2’ was not declared in this scope
        unw_init_local2(&cursor, &context, UNW_INIT_SIGNAL_FRAME);
        ^~~~~~~~~~~~~~~
src.cxx:8:8: note: suggested alternative: ‘unw_init_local’
        unw_init_local2(&cursor, &context, UNW_INIT_SIGNAL_FRAME);
        ^~~~~~~~~~~~~~~
        unw_init_local
ninja: build stopped: subcommand failed.

Source file was:

    #include <ucontext.h>
    #define UNW_LOCAL_ONLY
    #include <libunwind.h>
    int main () {
       ucontext_t context;
       unw_cursor_t cursor;
       unw_init_local2(&cursor, &context, UNW_INIT_SIGNAL_FRAME);
       return 0;
    }