Have libunwind.h export the version number via manifest constants
UNW_VERSION_MAJOR, UNW_VERSION_MINOR, and UNW_VERSION_EXTRA.
The last one is a free-form string.  For example, version
0.98-alpha would be represented as UNW_VERSION_MAJOR=0,
UNW_VERSION_MINOR=98, UNW_VERSION_EXTRA=-alpha (note: no quotes;
you'll have to stringify this value if necessary).

BKrev: 413f431cEdq7duwyl7TVLCIV4D7M9Q