Create $SYSCONFDIR/rpm directory on "make install"

Somehow nobody has noticed we don't create the per-host configuration
directory on install, probably didn't even in autoconf days.
We'll need this now in the next commits though.

CMakeLists.txt

@@ -479,6 +479,7 @@ endif()
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/rpm.pc DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)
 install(DIRECTORY DESTINATION ${RPM_CONFIGDIR}/lua)
 install(DIRECTORY DESTINATION ${RPM_CONFIGDIR}/macros.d)
+install(DIRECTORY DESTINATION ${CMAKE_INSTALL_FULL_SYSCONFDIR}/rpm)
 install(FILES CONTRIBUTING.md COPYING CREDITS INSTALL README TYPE DOC)
 
 add_custom_target(ChangeLog