fix make distcheck

[garlick]

I'm not sure how travis-ci is succeeding, but on my desktop, powerman's make distcheck is failing due to hardwired install paths:

make[3]: Entering directory '/home/garlick/proj/powerman/powerman-66cedb7/_build/sub/scripts'
make[3]: Nothing to be done for 'install-exec-am'.
 /bin/mkdir -p '/lib/systemd/system'
 /usr/bin/install -c ../../../scripts/powerman.service '/lib/systemd/system'
/usr/bin/install: cannot create regular file '/lib/systemd/system/powerman.service': Permission denied
Makefile:392: recipe for target 'install-systemdsystemunitSCRIPTS' failed
make[3]: *** [install-systemdsystemunitSCRIPTS] Error 1
make[3]: Leaving directory '/home/garlick/proj/powerman/powerman-66cedb7/_build/sub/scripts'
Makefile:482: recipe for target 'install-am' failed
make[2]: *** [install-am] Error 2
make[2]: Leaving directory '/home/garlick/proj/powerman/powerman-66cedb7/_build/sub/scripts'
Makefile:452: recipe for target 'install-recursive' failed
make[1]: *** [install-recursive] Error 1
make[1]: Leaving directory '/home/garlick/proj/powerman/powerman-66cedb7/_build/sub'
Makefile:658: recipe for target 'distcheck' failed
make: *** [distcheck] Error 1

[garlick]

Fails on RHEL 7.3 also:

$ make distcheck
[snip]
Making install in scripts
make[2]: Entering directory `/g/g0/garlick/powerman/powerman-66cedb7/_build/scripts'
make[3]: Entering directory `/g/g0/garlick/powerman/powerman-66cedb7/_build/scripts'
make[3]: Nothing to be done for `install-exec-am'.
 /usr/bin/mkdir -p '/usr/lib/systemd/system'
 /usr/bin/install -c ../../scripts/powerman.service '/usr/lib/systemd/system'
/usr/bin/install: cannot remove '/usr/lib/systemd/system/powerman.service': Read-only file system
make[3]: *** [install-systemdsystemunitSCRIPTS] Error 1
make[3]: Leaving directory `/g/g0/garlick/powerman/powerman-66cedb7/_build/scripts'
make[2]: *** [install-am] Error 2
make[2]: Leaving directory `/g/g0/garlick/powerman/powerman-66cedb7/_build/scripts'
make[1]: *** [install-recursive] Error 1
make[1]: Leaving directory `/g/g0/garlick/powerman/powerman-66cedb7/_build'
make: *** [distcheck] Error 1

[garlick]

Just tested on Ubuntu 22.04 LTS and it worked:

==============================================================
powerman-v2.3.27-30-gad3a31a archives ready for distribution: 
powerman-v2.3.27-30-gad3a31a.tar.gz
==============================================================