Factor code for building upc-related link options
Add UPC link support for NetBSD

This is sufficient to compile and link a UPC "Hello, World!" program
with clang-upc on NetBSD-6.1.4/amd64.  However, the runtime fails at
startup.

BUILD NOTE:
Additionally, I needed to pass -DLIBUPC_ENABLE_BACKTRACE=FALSE to
cmake due to a lack of backtrace() and backtrace_symbols() in
NetBSD-6.  They are introduced in NetBSD-7.0, but in a separate
libexecinfo which will some require additional logic to link
conditionally.
Add UPC link support for FreeBSD

This is sufficient to compile and link a UPC "Hello, World!" program
with clang-upc on FreeBSD-10.0-RELEASE-p4/amd64.  However, the runtime
fails at startup.

BUILD NOTE:
Additionally, I needed to pass -DLIBUPC_ENABLE_BACKTRACE=FALSE to
cmake.  While backtrace() and backtrace_symbols() are present in
FreeBSD-10.0 (though missing in earlier releases), are in a separate
libexecinfo.  Some additional logic could/should be added to link
with "-lexecinfo" when an a FreeBSD erlease with backtrace support.
Add UPC link support for OpenBSD

This is sufficient to compile and link a UPC "Hello, World!" program
with clang-upc on OpenBSD-5.5/amd64.  However, the runtime fails at
startup.

BUILD NOTE:
Additionally, I needed to pass -DLIBUPC_ENABLE_BACKTRACE=FALSE to
cmake, because OpenBSD lacks backtrace() and backtrace_symbols().
Merge branch 'bsd-support'

* bsd-support:
  Add UPC link support for OpenBSD
  Add UPC link support for FreeBSD
  Add UPC link support for NetBSD
  Factor code for building upc-related link options