Allow user to specify custom prefix for resource files.

[cireu]

This PR is successor of #9, removes all hardcoded path in build system.

There's still some hardcoded path in C++ source. IMO, there's better way to fix them, maybe add new function to search those resources(e.g. search environment variable first, then search FHS path, finally search builtin path)

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• CMakeLists.txt: include GNUInstallDirs.

• cpufreq/CMakeLists.txt: Replace hardcoded path with ${CMAKE_INSTALL_XXXXX} variables.

• session/CMakeLists.txt: Ditto.

• shutdown-ui/CMakeLists.txt: Ditto.

• setting-daemon/CMakeLists.txt: Ditto.

• screen-brightness/CMakeLists.txt: Ditto.

• polkit-agent/CMakeLists.txt: Ditto.

• screen-brightness/org.cutefish.brightness.pkexec.policy:

Rename to file with ".in" extension. Replace hardcoded path with @CMAKE_INSTALL_FULL_XXXXX@ variables.

• cpufreq/org.cutefish.cpufreq.pkexec.policy: Ditto.

[reionwong]

Thank you for your contribution :)