Use GNUInstallDirs to set installation directories

[Zopolis4]

Allow the user to override directories via typical options if necessary, otherwise use the GNU defaults.

[dpasukhi]

Dear @Zopolis4

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[dpasukhi]

I didn't review, but all variables can be overrided. You just change the way of overriding. You can override all your changed values directly by providing arguments to cmake or override after first configuration.

[Zopolis4]

Using GNUInstallDirs allows the right directories to be picked automatically in some cases, such as lib64 rather than lib.