Determining if the function pthread_create exists in the pthreads failed with the following output:
Change Dir: /home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/librtlsdr-v0.5.4/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_77f9f/fast"
make[1]: Entering directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/librtlsdr-v0.5.4/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_77f9f.dir/build.make CMakeFiles/cmTC_77f9f.dir/build
make[2]: Entering directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/librtlsdr-v0.5.4/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_77f9f.dir/CheckFunctionExists.c.o
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/bin/i686-linux-gcc  --sysroot=/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/i686-buildroot-linux-uclibc/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -static -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG   -o CMakeFiles/cmTC_77f9f.dir/CheckFunctionExists.c.o   -c /usr/share/cmake-3.5/Modules/CheckFunctionExists.c
Linking C executable cmTC_77f9f
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_77f9f.dir/link.txt --verbose=1
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/bin/i686-linux-gcc  --sysroot=/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/i686-buildroot-linux-uclibc/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -static -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG  -static  CMakeFiles/cmTC_77f9f.dir/CheckFunctionExists.c.o  -o cmTC_77f9f -rdynamic -lpthreads /home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/i686-buildroot-linux-uclibc/sysroot/usr/lib/libusb-1.0.a 
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/opt/ext-toolchain/bin/../lib/gcc/i686-buildroot-linux-uclibc/6.4.0/../../../../i686-buildroot-linux-uclibc/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_77f9f.dir/build.make:98: recipe for target 'cmTC_77f9f' failed
make[2]: *** [cmTC_77f9f] Error 1
make[2]: Leaving directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/librtlsdr-v0.5.4/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_77f9f/fast' failed
make[1]: *** [cmTC_77f9f/fast] Error 2
make[1]: Leaving directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/librtlsdr-v0.5.4/CMakeFiles/CMakeTmp'