Make install tries to put client service file in the root space, despite giving --prefix=/different/local/path/than/root to the configure script while compiling. IMO this is not the expected behaviour. With --prefix option, the compiled client binaries should be put in the /different/local/path/than/root folder, and it let user start the client manually, than as a system service by putting it in the root space.

$ ./configure --prefix=/different/local/path/than/root

$ make install

/usr/bin/install: cannot create regular file '/usr/lib/systemd/system/boinc-client.service': Permission denied
make[4]: *** [Makefile:562: install-exec-hook] Error 1
make[4]: Leaving directory '/work/boinc/client/scripts'
make[3]: *** [Makefile:503: install-exec-am] Error 2
make[3]: Leaving directory '/work/boinc/client/scripts'
make[2]: *** [Makefile:453: install-am] Error 2
make[2]: Leaving directory '/work/boinc/client/scripts'
make[1]: *** [Makefile:649: install-recursive] Error 1
make[1]: Leaving directory '/work/boinc'
make: *** [Makefile:951: install] Error 2

Is this a known bug or should this get filed as a new bug?

I would liked you to put that through to Github immediately, yes. No need to dawdle here. :)
https://github.com/BOINC/boinc/issues

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