Cstubs.write_c now includes headers that are needed in generated code.
Move cstubs_internals.h to the ctypes directory.

'make install' installs cstubs_internals.h under ctypes/, and having
it in the equivalent place in the source tree makes in-tree builds
easier.
Merge pull request #154 from yallop/cstubs-include-headers

Make cstubs-generated code more self-contained
Flush output streams when generating inverted C stubs.
Share fresh variable name generation across functions.
Handle CAMLreturnT(void, ...)
Include cstubs_internals.h in the inverted stubs output