Rename header guards to add c++ support

[DictumMortuum]

If you expect your code will ever be used from C++ (which a lot of C code is) then you should not use any identifiers with a double underscore.

#ifndef BITMAP__
    //        ^^   May cause problems in C++

As this is reserved for the implementation in C++. Also the term BITMAP is very generic and you are very likely to clash with other libraries try and make it more unique to your project.

[DictumMortuum]

Applies to external sort.