Restrict use of C++ language features to bare minimum

MACSio is almost entirely C. But, there are a few places where the keyword _mutable_ is used for members of a struct that is used in a mostly `const` setting. Also, plugins are loaded during executable load time via the use of [non-constant, static initializers which are not supported in C but are in C++](https://stackoverflow.com/questions/13785250/definition-of-global-variables-using-a-non-constant-initializer).

We should find a way to restrict all this to a single C++ source file having all other files be C code and compiled with a C compiler. Currently, we are using C++ compiler everywhere.

Alternatively, we can keep use of C++ compiler everywhere but we should then change the source file suffixes to be either `.C` or `.cxx`.

Start by inspecting the source tree and build configuration to locate the mutable struct members, static initializers, and compiler settings described in the issue. Determine whether the project can isolate C++ to one source file or should rename all C++ sources with .C or .cxx suffixes. Done means the chosen approach is applied consistently and the project builds successfully.

Written by the indexing model from the issue text.