BUILD: simplify the date and version retrieval in the makefile

The makefile currently uses some complex and non-always portable
methods to retrieve the date and version (eg: linux's date command).

For the date, we can use git log -1 --pretty=format:%ci instead of
date+sed. For the version, it's easier and safer to count single log
lines.

Note that the VERSION variable was wrong since it could contain the
version+subversion instead of just the version. This is now fixed by
adding --abbrev=0 in describe.