More robust common_config_file_iterator

common_config_file_iterator::common_config_file_iterator has a little problem: One of the args is std::set, which is incompatible between libc++ and libstdc++. As a result, the function, if compiled with {GCC, Clang}, is incompatible with {Clang, GCC). This can be fixed without breaking anything by moving it to the header part and declaring it static. What are your opinions about it?

Locate the common_config_file_iterator declaration and definition, then inspect how its std::set argument crosses the compiled interface. Compare builds with GCC and Clang and verify that moving the implementation to the header and declaring it static preserves compatibility. Done means the iterator can be used across those compiler combinations without the reported incompatibility.

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