Starting with version 3.39 the C-interface has been moved in directory c_inter. The discussion which follows basically explains the content of the files c.c c.h and c.pl in this directory.
Once BinProlog is correctly installed as file "bp" somewhere in your path along with the libraries bp.o and wam.o which should be in directory ../src and you have a C-compiler around, you can simply do make in directory c_inter to create a standalone application which integrates the functionality of the C-code in file c.c.
To test the resulting standalone executable c_binpro, try out the demo c_test defined in file c.pl. Start running it by typing c_binpro and then type c_test. at the Prolog prompt. To avoid the prompt and directly launch the application add
main:-c_test.in file c.pl before typing make.