Determining if the pthread_create exist failed with the following output:
Change Dir: /home/buildroot/build/instance-0/output/build/gnuradio-3.7.13.3/buildroot-build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_f7cf0/fast"
make[1]: Entering directory '/home/buildroot/build/instance-0/output/build/gnuradio-3.7.13.3/buildroot-build/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_f7cf0.dir/build.make CMakeFiles/cmTC_f7cf0.dir/build
make[2]: Entering directory '/home/buildroot/build/instance-0/output/build/gnuradio-3.7.13.3/buildroot-build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_f7cf0.dir/CheckSymbolExists.c.o
/home/buildroot/build/instance-0/output/host/bin/mipsel-linux-gcc --sysroot=/home/buildroot/build/instance-0/output/host/mipsel-buildroot-linux-gnu/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -std=gnu99 -fvisibility=hidden -Wsign-compare -Wall -Wno-uninitialized   -DNDEBUG   -o CMakeFiles/cmTC_f7cf0.dir/CheckSymbolExists.c.o   -c /home/buildroot/build/instance-0/output/build/gnuradio-3.7.13.3/buildroot-build/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_f7cf0
/home/buildroot/build/instance-0/output/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_f7cf0.dir/link.txt --verbose=1
/home/buildroot/build/instance-0/output/host/bin/mipsel-linux-gcc --sysroot=/home/buildroot/build/instance-0/output/host/mipsel-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -std=gnu99 -fvisibility=hidden -Wsign-compare -Wall -Wno-uninitialized   -DNDEBUG    -rdynamic CMakeFiles/cmTC_f7cf0.dir/CheckSymbolExists.c.o  -o cmTC_f7cf0 
CMakeFiles/cmTC_f7cf0.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text.startup+0x8): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_f7cf0.dir/build.make:97: recipe for target 'cmTC_f7cf0' failed
make[2]: *** [cmTC_f7cf0] Error 1
make[2]: Leaving directory '/home/buildroot/build/instance-0/output/build/gnuradio-3.7.13.3/buildroot-build/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_f7cf0/fast' failed
make[1]: *** [cmTC_f7cf0/fast] Error 2
make[1]: Leaving directory '/home/buildroot/build/instance-0/output/build/gnuradio-3.7.13.3/buildroot-build/CMakeFiles/CMakeTmp'

File /home/buildroot/build/instance-0/output/build/gnuradio-3.7.13.3/buildroot-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: /home/buildroot/build/instance-0/output/build/gnuradio-3.7.13.3/buildroot-build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_6f640/fast"
make[1]: Entering directory '/home/buildroot/build/instance-0/output/build/gnuradio-3.7.13.3/buildroot-build/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_6f640.dir/build.make CMakeFiles/cmTC_6f640.dir/build
make[2]: Entering directory '/home/buildroot/build/instance-0/output/build/gnuradio-3.7.13.3/buildroot-build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_6f640.dir/CheckFunctionExists.c.o
/home/buildroot/build/instance-0/output/host/bin/mipsel-linux-gcc --sysroot=/home/buildroot/build/instance-0/output/host/mipsel-buildroot-linux-gnu/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -std=gnu99 -fvisibility=hidden -Wsign-compare -Wall -Wno-uninitialized -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG   -o CMakeFiles/cmTC_6f640.dir/CheckFunctionExists.c.o   -c /home/buildroot/build/instance-0/output/host/share/cmake-3.8/Modules/CheckFunctionExists.c
Linking C executable cmTC_6f640
/home/buildroot/build/instance-0/output/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_6f640.dir/link.txt --verbose=1
/home/buildroot/build/instance-0/output/host/bin/mipsel-linux-gcc --sysroot=/home/buildroot/build/instance-0/output/host/mipsel-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -std=gnu99 -fvisibility=hidden -Wsign-compare -Wall -Wno-uninitialized -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG    -rdynamic CMakeFiles/cmTC_6f640.dir/CheckFunctionExists.c.o  -o cmTC_6f640 -lpthreads 
/home/buildroot/build/instance-0/output/host/opt/ext-toolchain/bin/../lib/gcc/mipsel-buildroot-linux-gnu/6.4.0/../../../../mipsel-buildroot-linux-gnu/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_6f640.dir/build.make:97: recipe for target 'cmTC_6f640' failed
make[2]: *** [cmTC_6f640] Error 1
make[2]: Leaving directory '/home/buildroot/build/instance-0/output/build/gnuradio-3.7.13.3/buildroot-build/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_6f640/fast' failed
make[1]: *** [cmTC_6f640/fast] Error 2
make[1]: Leaving directory '/home/buildroot/build/instance-0/output/build/gnuradio-3.7.13.3/buildroot-build/CMakeFiles/CMakeTmp'