Make include guards standard-compliant and unique.

Include guards prevent the multiple inclusion of header files only correctly if the chosen identifiers are different from others with such a functionality.
The probability for name clashes can be reduced by the addition of the project name as a prefix and a kind of universally unique identifier as a suffix.

This is a companion update suggestion for the issue "[reserved identifier violation](https://github.com/fix8/fix8/issues/49 "Check standard-compliance of identifiers")".