Revert "README: Don't hardcode /etc in cmake fallback dir"

This reverts commit 2c5efeec3a.

Dunno if there's a standard way to refer to /etc in cmake after all.

README

@@ -146,8 +146,7 @@ Q. I author/maintain package X and would like to maintain my own
        message(STATUS
          "Using bash completion dir ${BASH_COMPLETION_COMPLETIONSDIR}")
      else()
-       set (BASH_COMPLETION_COMPLETIONSDIR
-         "${CMAKE_INSTALL_SYSCONFDIR}/bash_completion.d")
+       set (BASH_COMPLETION_COMPLETIONSDIR "/etc/bash_completion.d")
        message (STATUS
          "Using fallback bash completion dir ${BASH_COMPLETION_COMPLETIONSDIR}")
      endif()