Compilation error when using older compilers due to missing std::filesystem

[nmanne-anlgov]

When compiling this library using GCC 7.5, there is a compilation error due to missing std::filesystem. GCC 7.5 implements the C++17 standard but doesn't contain the filesystem implementation.

There is a flag LIBEVSE_SECURITY_USE_BOOST_FILESYSTEM, that is supposed to make the project use boost:filesystem instead of std::filesystem, but the implementation of this flag seems incomplete and the code has several usages of std::filesystem instead of the alias fs.

[nmanne-anlgov]

I created a PR (#129) to fix this issue.

[hikinggrass]

thanks for the fix, merged #129