Note:

  - The "normal" way to inject CFLAGS (and CPPFLAGS) into nx-X11 builds is copying those
    options over into the variable CDEBUGFLAGS.
  - LDFLAGS have to be handed to nx-X11 via LOCAL_LDFLAGS.

 This change also includes a slight change in the nx-X11 build order.

 Old build order:

  Main Makefile:

      - [...]
      - libNX_X11
        + implicitly building nxcomp
        + implicitly building nxcompext
      - [...]
      - nxagent
        + implicitly building nxcompshad
      - [...]

 New build ordner:

  Main Makefile:

      - [...]
      - nxcomp
      - setup nx-X11 build env
        + cd nx-X11 && make BuildEnv
      - nxcompext
      - nxcompshad
      - nx-X11/lib/*
      - [...]
      - nxagent
      - [...]

 Fixes ArcticaProject/nx-libs#141
 Fixes X2GoBTS#84