help.m4 includes incorrect message for apt-get

In common/autoconf/help.m4, there is a line in apt_help() as follows:

---
      PKGHANDLER_COMMAND="sudo apt-get install libX11-dev libxext-dev libxrender-dev libxtst-dev libxt-dev" ;;
---

"libX11-dev" should be all lowercase "libx11-dev", otherwise the actual apt-get invocation will fail with the following error:

"E: Unable to locate package libX11-dev"