platform: Make SOVERSION explicit

Just using $MAJOR.$MINOR is not a clever idea -- we'd like to have the
5.2 branch with versions like (5.1.40 for 'not released 5.2' or 5.1.80
for '5.2 rc' not have 51 as SOVERSION, but indeed 52).

(cherry picked from commit b1793437e29bdc775386427190112d42b07b0eeb)