Add linuxcnc-python script to system install

Tests running against system install need `linuxcnc-python` in
`$PATH`.

The system-installed `stepconf` script uses it, too, so this
particular commit is a fix rather than a hack.

src/Makefile

@@ -629,6 +629,7 @@ install-kernel-indep: install-dirs
 	$(EXE) ../scripts/halrun $(DESTDIR)$(bindir)
 	$(EXE) ../scripts/halcmd_twopass $(DESTDIR)$(bindir)
 	$(EXE) ../scripts/haltcl $(DESTDIR)$(bindir)
+	$(EXE) ../scripts/linuxcnc-python $(DESTDIR)$(bindir)
 	$(EXE) ../scripts/simulate_probe $(DESTDIR)$(bindir)
 	$(EXE) ../scripts/sim_pin $(DESTDIR)$(bindir)
 	$(FILE) ../*.png ../*.gif $(DESTDIR)$(datadir)/linuxcnc