Build: Append git revision hash to version

Build: Append git revision hash to version

Append the git revision hash to the libsigrok package version,
unless HEAD exactly matches a release tag.  Note that this does
not affect the version known to autoconf -- e.g. source tarballs
created by make dist will not receive a revision suffix.

Changes to git HEAD automatically trigger a reconfiguration.
Uncommitted changes do not, which is why I left out the -dirty
suffix.