Determining if the pthread_create exist failed with the following output:
Change Dir: /var/tmp/portage/app-i18n/librime-1.2.9/work/librime-1.2.9_build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/gmake cmTC_9957a/fast 
/usr/bin/gmake -f CMakeFiles/cmTC_9957a.dir/build.make CMakeFiles/cmTC_9957a.dir/build
gmake[1]: Entering directory '/var/tmp/portage/app-i18n/librime-1.2.9/work/librime-1.2.9_build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_9957a.dir/CheckSymbolExists.c.o
/usr/bin/x86_64-pc-linux-gnu-gcc   -O2 -pipe -march=native    -o CMakeFiles/cmTC_9957a.dir/CheckSymbolExists.c.o   -c /var/tmp/portage/app-i18n/librime-1.2.9/work/librime-1.2.9_build/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_9957a
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_9957a.dir/link.txt --verbose=1
/usr/bin/x86_64-pc-linux-gnu-gcc -O2 -pipe -march=native   -Wl,-O1 -Wl,--as-needed  -rdynamic CMakeFiles/cmTC_9957a.dir/CheckSymbolExists.c.o  -o cmTC_9957a 
/usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/cmTC_9957a.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_9957a.dir/build.make:87: cmTC_9957a] Error 1
gmake[1]: Leaving directory '/var/tmp/portage/app-i18n/librime-1.2.9/work/librime-1.2.9_build/CMakeFiles/CMakeTmp'
gmake: *** [Makefile:121: cmTC_9957a/fast] Error 2

File /var/tmp/portage/app-i18n/librime-1.2.9/work/librime-1.2.9_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
}

Determining if the function pthread_create exists in the pthreads failed with the following output:
Change Dir: /var/tmp/portage/app-i18n/librime-1.2.9/work/librime-1.2.9_build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/gmake cmTC_56275/fast 
/usr/bin/gmake -f CMakeFiles/cmTC_56275.dir/build.make CMakeFiles/cmTC_56275.dir/build
gmake[1]: Entering directory '/var/tmp/portage/app-i18n/librime-1.2.9/work/librime-1.2.9_build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_56275.dir/CheckFunctionExists.c.o
/usr/bin/x86_64-pc-linux-gnu-gcc   -O2 -pipe -march=native -DCHECK_FUNCTION_EXISTS=pthread_create   -o CMakeFiles/cmTC_56275.dir/CheckFunctionExists.c.o   -c /usr/share/cmake/Modules/CheckFunctionExists.c
Linking C executable cmTC_56275
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_56275.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_56275.dir/CheckFunctionExists.c.o  -o cmTC_56275 -lpthreads 
/usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
gmake[1]: *** [CMakeFiles/cmTC_56275.dir/build.make:87: cmTC_56275] Error 1
gmake[1]: Leaving directory '/var/tmp/portage/app-i18n/librime-1.2.9/work/librime-1.2.9_build/CMakeFiles/CMakeTmp'
gmake: *** [Makefile:121: cmTC_56275/fast] Error 2