Get Graphviz libraries directly from the pkgconfig/libgvc.pc file, or from a user
specified list, as pkg-config is not available by default on OS X.
Force move constructors and assignment to default for Interval and MemoryArea, to
satisfy building on OS X.
Merge updated upstream with local branch at CSSE.
Correct initialize UErrorCode to U_ZERO_ERROR.
Fix build on OS X using libc++ (there's currently an ambiguity bug when using
std::async, but it will be gone in the next release,  hence all the ugly
conditional compilation).
Fix build on OS X.