Add a high level version number for SPIRV-Tools, beginning
with v2016.0-dev. The README describes the format of the
version number.
The high level version number is extracted from the CHANGES
file. That works around:
- stale-bait for when we don't add tags to the repository
- our inability to add tags to the repository
Option --version causes spirv-as, spirv-dis, and spirv-val to
show the high level version number.
Add spvSoftwareVersionString to return the C-string for
the high level version number.
Add spvSoftwareVersionDetailsString() so that clients can get
more information if they want to.
Also allows us to clean up the uses in the tool executables files,
so now only one file includes build-version.inc.
Move the update-build-version logic to the only
CMakeLists file that needs it.
The update build version script takes a new argument
to name the output file.
