BUILD: makefile: use :space: instead of digits to count commits

BUILD: makefile: use :space: instead of digits to count commits

The 'tr' command on Solaris doesn't conform to POSIX and requires
brackets around ranges. So the sequence '0-9' is understood as the
3 characters '0', '-', and '9'. This causes tagged versions (those
with no commit after the last commit) to be numberred as an empty
string, resulting in an error being reported while computing the
version number.

All implementations support '[:space:]' to delete heading spaces,
so let's use this instead.

This may be backported to all stable versions.