cpupower: split unitdir from libdir in Makefile

Improve the installation procedure for the systemd service unit
'cpupower.service', to be more flexible. Some distros install libraries
to /usr/lib64/, but systemd service units have to be installed to
/usr/lib/systemd/system: as a consequence, the installation procedure
should not assume that systemd service units can be installed to
${libdir}/systemd/system ...
Define a dedicated variable ("unitdir") in the Makefile.

Link: https://lore.kernel.org/linux-pm/[e-mail]/T/#m0601940ab439d5cbd288819d2af190ce59e810e6

Fixes: 9c70b779ad91 ("cpupower: add a systemd service to run cpupower")

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Francesco Poli (wintermute) <[e-mail]>
Tested-by: Thorsten Leemhuis <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/power/cpupower/Makefile

···
 mandir ?=	/usr/man
 libdir ?=	/usr/lib
 libexecdir ?=	/usr/libexec
+unitdir ?=	/usr/lib/systemd/system
 includedir ?=	/usr/include
 localedir ?=	/usr/share/locale
 docdir ?=       /usr/share/doc/packages/cpupower
···
 	$(INSTALL_DATA) cpupower-service.conf '$(DESTDIR)${confdir}'
 	$(INSTALL) -d $(DESTDIR)${libexecdir}
 	$(INSTALL_PROGRAM) cpupower.sh '$(DESTDIR)${libexecdir}/cpupower'
-	$(INSTALL) -d $(DESTDIR)${libdir}/systemd/system
-	sed 's|___CDIR___|${confdir}|; s|___LDIR___|${libexecdir}|' cpupower.service.in > '$(DESTDIR)${libdir}/systemd/system/cpupower.service'
-	$(SETPERM_DATA) '$(DESTDIR)${libdir}/systemd/system/cpupower.service'
+	$(INSTALL) -d $(DESTDIR)${unitdir}
+	sed 's|___CDIR___|${confdir}|; s|___LDIR___|${libexecdir}|' cpupower.service.in > '$(DESTDIR)${unitdir}/cpupower.service'
+	$(SETPERM_DATA) '$(DESTDIR)${unitdir}/cpupower.service'
 
 install-man:
 	$(INSTALL_DATA) -D man/cpupower.1 $(DESTDIR)${mandir}/man1/cpupower.1
···
 	- rm -f $(DESTDIR)${bindir}/utils/cpupower
 	- rm -f $(DESTDIR)${confdir}cpupower-service.conf
 	- rm -f $(DESTDIR)${libexecdir}/cpupower
-	- rm -f $(DESTDIR)${libdir}/systemd/system/cpupower.service
+	- rm -f $(DESTDIR)${unitdir}/cpupower.service
 	- rm -f $(DESTDIR)${mandir}/man1/cpupower.1
 	- rm -f $(DESTDIR)${mandir}/man1/cpupower-frequency-set.1
 	- rm -f $(DESTDIR)${mandir}/man1/cpupower-frequency-info.1