First, thanks a lot for publishing this! Fixed my CI build on gitlab, which all of a sudden was enabled for my repo. But seriously, why isn't a buildpack for C already a standard component in this heroku thing?
Now for the issue: Shouldn't this be called heroku-buildpack-make or something? There is nothing specific to C in here, and it could work for all kind of software building that uses a makefile.
A proper C buildpack should look for *.c files and if there is only one example.c it could run "make example" and make will do the right thing even without a Makefile or configure.