- remove extern declaration in source file, use header instead
- add inclusion of header defining the functions of the file to
get the compiler to cross-check them
- marked static the functions that should not be visible ouside
their file
Considering the number of headers we have, it is a good idea to
avoid possible problems. For details, you may read:
http://en.wikipedia.org/wiki/Include_guard
All headers should be ok now.
