Determining if the include file io.h exists failed with the following output:
Change Dir: /home/mark/buildroot-test/instance-0/output/build/cmocka-1.1.5/buildroot-build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_b21ec/fast"
make[1]: Entering directory '/home/mark/buildroot-test/instance-0/output/build/cmocka-1.1.5/buildroot-build/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_b21ec.dir/build.make CMakeFiles/cmTC_b21ec.dir/build
make[2]: Entering directory '/home/mark/buildroot-test/instance-0/output/build/cmocka-1.1.5/buildroot-build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_b21ec.dir/CheckIncludeFile.c.o
/home/mark/buildroot-test/instance-0/output/host/bin/riscv64-buildroot-linux-musl-gcc --sysroot=/home/mark/buildroot-test/instance-0/output/host/riscv64-buildroot-linux-musl/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG -fPIE   -o CMakeFiles/cmTC_b21ec.dir/CheckIncludeFile.c.o   -c /home/mark/buildroot-test/instance-0/output/build/cmocka-1.1.5/buildroot-build/CMakeFiles/CMakeTmp/CheckIncludeFile.c
/home/mark/buildroot-test/instance-0/output/build/cmocka-1.1.5/buildroot-build/CMakeFiles/CMakeTmp/CheckIncludeFile.c:1:10: fatal error: io.h: No such file or directory
 #include <io.h>
          ^~~~~~
compilation terminated.
make[2]: *** [CMakeFiles/cmTC_b21ec.dir/build.make:66: CMakeFiles/cmTC_b21ec.dir/CheckIncludeFile.c.o] Error 1
make[2]: Leaving directory '/home/mark/buildroot-test/instance-0/output/build/cmocka-1.1.5/buildroot-build/CMakeFiles/CMakeTmp'
make[1]: *** [Makefile:121: cmTC_b21ec/fast] Error 2
make[1]: Leaving directory '/home/mark/buildroot-test/instance-0/output/build/cmocka-1.1.5/buildroot-build/CMakeFiles/CMakeTmp'