The purpose of build.h is so that C developers do not have to struggle with learning Make, CMake, Meson, etc. and can just configure their build system in C.
All you have to do is place the build.h file in your C project directory, and write a build.c file to configure it. To build the project, all you need to do is compile build.c, which can be done with:
gcc -o build build.cor, if you have make installed, you can do:
make build(The above command does not need a Makefile, it uses make's implicit rules.)
Running ./build will build the entire project.