Create a tutorial for working with C libraries

[joshring]

This is more of a brain dump each of these are likely an issue

• C header only libraries - usually do an example with addition or similar
• Larger C libraries, writing C3 bindings make sure to link to C interop's gotchas!
• Working with Static C libraries
• Tips for porting C libraries

[tomaskallup]

It would be nice to also include a section about working with c-sources in project.

I was able to make a quick example, storing it here for reference.
In project folder, I created c-src folder, to put any C libraries in there.

.
└── c-src
    └── mylib
        ├── include
        │   └── mylib.h
        └── src
            └── mylib.c

mylib.c has and include for the header #include "mylib.h"

This is then glued together via project.json options:

{
  "c-sources": [ "./c-src/mylib/src/" ],
  "c-include-dirs": [ "./c-src/mylib/include" ],
}

When running c3c build these options are just passed to cc:

cc -I ./c-src/mylib/include/ -fPIE -fPIC -c ./c-src/mylib/src/*.c -o build/tmp/tmp_c_compile/mylib.o

And the result is just linked to the C3 code. This means that any functions which we want to use from mylib need to be declared in C3:

extern fn void do_stuff(int val);

fn int main(String[] args)
{
  do_stuff(0);
  return 0;
}