Allow build on HP-UX 11.X

Patch from Richard Lloyd (slightly edited from me), following the list
of changes as described by the author:

src/Makefile:
- Added PREFIX and BINDIR for the install: rule.
- Added a "make hpux" line to the help: rule.
- Added "make test"/"make check" rule that runs the $(PGOBENCH) command.
- "make clean" now additionally removes core and bench.txt.
- Added an hpux: rule.
- Added an install: rule to mkdir $(BINDIR), copy $(EXE) to $(BINDIR) and
 then strip it.
- "make strip" now ensures that $(EXE) is built first before trying to
 strip it.
- Hide errors and output from the g++ command used by the .depend: rule and
 then touch .depend in case g++ isn't available.
- Hide errors from the "include .depend" in case .depend doesn't exist
 (e.g. directly after a "make clean").

src/book.cpp and src/book.h:
- HP-UX's aCC really didn't like the const keywords used for the
 Book::file_name() definitions, so they were removed. I checked that this
 didn't affect a Linux build and it was still fine.

src/misc.cpp:
- HP-UX uses <sys/pstat.h> and pstat_getdynamic() to determine the number of
 CPU cores, so added conditional code for that (if pstat_getdynamic() fails,
 set the number of cores to 1).

src/tt.cpp:
- <xmmintrin.h> and _mm_prefetch() seem highly specific to the Intel x86(_64)
 and gcc platforms - neither exist in HP-UX, so conditionally avoid that
 code in HP-UX's case. Perhaps some sort of define is needed here
 such as -DHAS_MM_PREFETCH that could be #ifdef'ed for instead?

Even after these changes, it's more convenient for HP-UX users to edit the
default: rule in the Makefile to run "$(MAKE) hpux" before they build
stockfish, but that's not a big deal if they're warned about that first (the
same applies to all other builds other than the standard "$(MAKE) gcc" one).

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/book.cpp

@@ -390,7 +390,7 @@ void Book::close() {
 /// Book::file_name() returns the file name of the currently active book,
 /// or the empty string if no book is open.
 
-const string Book::file_name() const {
+const string Book::file_name() { // Not const to compile on HP-UX 11.X
 
   return is_open() ? fileName : "";
 }