PDP1, PDP11, TX-0: Cleanup build messages on *nix platforms avoiding XKeycodeToKeysym deprecated messages.  Fix #58
Compiler suggested cleanup.
PDP1, PDP11, TX-0: Addition to cleanup build messages on *nix platforms avoiding XKeycodeToKeysym deprecated messages to also work when using clang (LLVM compiler).  Fix #58
makefile: Added build support for simulators which use the display library on host platforms which have X11 support not located in /usr/include.

Simulators affected: pdp1, pdp11 and tx-0
Host platforms which now build with display support: OS X
makefile: remote extraneous debug print statement.

OS X is not a useful platform for display (pdp1, pdp11 and tx-0 simulators) with the default Xcode provided APIs.
SCP: Updated documentation and help text
PDQ-3: Latest update from Holger Veit

- removed cycle counting in CPU, the sim_interval issue
- fixed the Console multiplexer logic; it now works with SET REMOTE TELNET= and SET CONSOLE TELNET=
- commented testhdt.sim
- changed run.cmd to try to execute either VStudio or mingw executable, in this order.
- changed printf messages to use sim_printf

Possible issues remaining:
- timer device polling, not yet debugged, the rate of 1.25MHz is likely still incorrect, so UCSD wall clock is surely wrong
- few instructions still to be trapped by opcode.debug - I haven't seen them in life code yet
- HD device still not yet finished; I'll pick this up again soon
makefile: Explicitely fail on old Haiku or bad arch

- We require a current Haiku release (post alpha4).
- We also don't support building with gcc2.
scp: avoid redefining MAX

On Haiku, we end up including <sys/param.h> from sim_sock.h which
defines MAX.
makefile: Add link library path (-L) when needed on platforms which don't readily locate the PCRE library
PDP11, VAX: Fix DEQNA emulation to properly indicate receive buffer length for the last segment when dropping incoming packets into multiple buffer descriptors.
AltairZ80: Fixed confusing call to memset
PDP11, VAX780: Fix DEUNA handling of incoming packets which are dropped into multiple receive buffers.

Make sure to properly support multiple receive buffers by correctly setting the data length read and the appropriate buffer descriptor status bits for each buffer descriptor that is used to hold a packet.
alpha: Compiler suggested cleanup