Determining if the include file pthread.h exists failed with the following output:
Change Dir: /home/buildroot/autobuild/run/instance-0/output-1/build/bullet-3.09/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_aa3b6/fast && make[1]: Entering directory `/home/buildroot/autobuild/run/instance-0/output-1/build/bullet-3.09/CMakeFiles/CMakeTmp'
/usr/bin/make  -f CMakeFiles/cmTC_aa3b6.dir/build.make CMakeFiles/cmTC_aa3b6.dir/build
make[2]: Entering directory `/home/buildroot/autobuild/run/instance-0/output-1/build/bullet-3.09/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_aa3b6.dir/CheckIncludeFile.c.o
/home/buildroot/autobuild/run/instance-0/output-1/host/bin/arm-linux-gcc --sysroot=/home/buildroot/autobuild/run/instance-0/output-1/host/arm-buildroot-linux-uclibcgnueabihf/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG   -o CMakeFiles/cmTC_aa3b6.dir/CheckIncludeFile.c.o   -c /home/buildroot/autobuild/run/instance-0/output-1/build/bullet-3.09/CMakeFiles/CMakeTmp/CheckIncludeFile.c
/home/buildroot/autobuild/run/instance-0/output-1/build/bullet-3.09/CMakeFiles/CMakeTmp/CheckIncludeFile.c:1:10: fatal error: pthread.h: No such file or directory
    1 | #include <pthread.h>
      |          ^~~~~~~~~~~
compilation terminated.
make[2]: *** [CMakeFiles/cmTC_aa3b6.dir/CheckIncludeFile.c.o] Error 1
make[2]: Leaving directory `/home/buildroot/autobuild/run/instance-0/output-1/build/bullet-3.09/CMakeFiles/CMakeTmp'
make[1]: *** [cmTC_aa3b6/fast] Error 2
make[1]: Leaving directory `/home/buildroot/autobuild/run/instance-0/output-1/build/bullet-3.09/CMakeFiles/CMakeTmp'