GSoC 2016: Raise error with CHPL_UNWIND=libunwind on darwin

[ghost]

This PR raises an error if CHPL_UNWIND is set to ‘libunwind’ on darwin.

Passed test/runtime/panzone with CHPL_UNWIND = none/libunwind/system on linux64.

[bradcray]

Hi @Panzone and @mppf:

Now when I set CHPL_UNWIND on my Mac and try to build I get:

mbp-bradc (master)$ export CHPL_UNWIND=libunwind
mbp-bradc (master)$ make
cd compiler && /Library/Developer/CommandLineTools/usr/bin/make
Traceback (most recent call last):
  File "/Users/bradc/chapel/util/printchplenv", line 253, in <module> _main()
  File "/Users/bradc/chapel/util/printchplenv", line 249, in _main print_mode(options.mode)
  File "/Users/bradc/chapel/util/printchplenv", line 79, in print_mode unwind = chpl_unwind.get()
  File "/Users/bradc/chapel/util/chplenv/utils.py", line 15, in memoize_wrapper cache[args] = func(*args)
  File "/Users/bradc/chapel/util/chplenv/chpl_unwind.py", line 26, in get " on Mac OS X. Use CHPL_UNWIND=system instead.")
ValueError: Using CHPL_UNWIND=libunwind is not supported on Mac OS X. Use CHPL_UNWIND=system instead.
/Users/bradc/chapel/make/Makefile.base:153: /Users/bradc/chapel/third-party/llvm/Makefile.include-: No such file or directory
make[2]: *** No rule to make target `/Users/bradc/chapel/third-party/llvm/Makefile.include-'.  Stop.
make[1]: *** [compiler] Error 2
make: *** [comprt] Error 2

I see that the error message is deep within there, but it's hard to see amongst the rest. Is this the intended behavior, a problem on my part, or a bug? Thanks.

[mppf]

@bradcray - can you point to an example where setting a CHPL_ variable in the wrong way gives an error message in a form you like?

[bradcray]

Not offhand. There used to be error messages related to bad TASKS/THREADS/COMM combinations, but I don't know that I've tried any of the bad combinations since a lot of this has been refactored, so don't know what happens there today. As I mentioned on mail yesterday, I'd also be open to a world in which it's legal from a Chapel perspective to set it to libunwind, but if libunwind can't be built under OS X (is that the reason?), we don't do anything to try to prevent you from running into that third-party issue yourself. (Or, in the future, if we roll libunwind into our build, perhaps the third-party Makefile could print out something short and sweet?).