introduce library version plus git hash

This exposes the library version and git head hash it was built from
into LWS_LIBRARY_VERSION and LWS_BUILD_HASH.

These are combined into a version string that's both printed as a
notice log by the library and made available to the app using a new
api lws_get_library_version().  The version looks like

 1.1 178d78c

Signed-off-by: Andy Green <andy.green@linaro.org>