Determining if the pthread_create exist failed with the following output:
Change Dir: /data/buildroot/buildroot-test/instance-0/output/build/i2pd-2.22.0/build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_cda77/fast"
make[1]: Entering directory '/data/buildroot/buildroot-test/instance-0/output/build/i2pd-2.22.0/build/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_cda77.dir/build.make CMakeFiles/cmTC_cda77.dir/build
make[2]: Entering directory '/data/buildroot/buildroot-test/instance-0/output/build/i2pd-2.22.0/build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_cda77.dir/CheckSymbolExists.c.o
/data/buildroot/buildroot-test/instance-0/output/host/bin/riscv32-linux-gcc --sysroot=/data/buildroot/buildroot-test/instance-0/output/host/riscv32-buildroot-linux-gnu/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG   -o CMakeFiles/cmTC_cda77.dir/CheckSymbolExists.c.o   -c /data/buildroot/buildroot-test/instance-0/output/build/i2pd-2.22.0/build/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_cda77
/data/buildroot/buildroot-test/instance-0/output/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_cda77.dir/link.txt --verbose=1
/data/buildroot/buildroot-test/instance-0/output/host/bin/riscv32-linux-gcc --sysroot=/data/buildroot/buildroot-test/instance-0/output/host/riscv32-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG    -rdynamic CMakeFiles/cmTC_cda77.dir/CheckSymbolExists.c.o  -o cmTC_cda77 
CMakeFiles/cmTC_cda77.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text.startup+0x0): undefined reference to `pthread_create'
CheckSymbolExists.c:(.text.startup+0x4): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/cmTC_cda77.dir/build.make:98: cmTC_cda77] Error 1
make[2]: Leaving directory '/data/buildroot/buildroot-test/instance-0/output/build/i2pd-2.22.0/build/CMakeFiles/CMakeTmp'
make[1]: *** [Makefile:126: cmTC_cda77/fast] Error 2
make[1]: Leaving directory '/data/buildroot/buildroot-test/instance-0/output/build/i2pd-2.22.0/build/CMakeFiles/CMakeTmp'

File /data/buildroot/buildroot-test/instance-0/output/build/i2pd-2.22.0/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 compiler accepts -pthread returned OFF instead of 2. The compiler had the following output:
Change Dir: /data/buildroot/buildroot-test/instance-0/output/build/i2pd-2.22.0/build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_3ce31/fast"
make[1]: Entering directory '/data/buildroot/buildroot-test/instance-0/output/build/i2pd-2.22.0/build/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_3ce31.dir/build.make CMakeFiles/cmTC_3ce31.dir/build
make[2]: Entering directory '/data/buildroot/buildroot-test/instance-0/output/build/i2pd-2.22.0/build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_3ce31.dir/CheckForPthreads.c.o
/data/buildroot/buildroot-test/instance-0/output/host/bin/riscv32-linux-gcc --sysroot=/data/buildroot/buildroot-test/instance-0/output/host/riscv32-buildroot-linux-gnu/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG   -o CMakeFiles/cmTC_3ce31.dir/CheckForPthreads.c.o   -c /data/buildroot/buildroot-test/instance-0/output/host/share/cmake-3.8/Modules/CheckForPthreads.c
Linking C executable cmTC_3ce31
/data/buildroot/buildroot-test/instance-0/output/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_3ce31.dir/link.txt --verbose=1
/data/buildroot/buildroot-test/instance-0/output/host/bin/riscv32-linux-gcc --sysroot=/data/buildroot/buildroot-test/instance-0/output/host/riscv32-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG    -rdynamic CMakeFiles/cmTC_3ce31.dir/CheckForPthreads.c.o  -o cmTC_3ce31 -pthread 
make[2]: Leaving directory '/data/buildroot/buildroot-test/instance-0/output/build/i2pd-2.22.0/build/CMakeFiles/CMakeTmp'
make[1]: Leaving directory '/data/buildroot/buildroot-test/instance-0/output/build/i2pd-2.22.0/build/CMakeFiles/CMakeTmp'