build: Rename LIBBSD_ABI to SOVERSION

This matches the semantics of the variable, and makes it independent of
the project, just as the package variables.

configure.ac

@@ -16,12 +16,12 @@ AM_INIT_AUTOMAKE(
 )
 AM_SILENT_RULES([yes])
 
-LIBBSD_ABI_MAJOR=0
-LIBBSD_ABI_MINOR=11
-LIBBSD_ABI_PATCH=8
+SOVERSION_MAJOR=0
+SOVERSION_MINOR=11
+SOVERSION_PATCH=8
 
-LIBBSD_ABI="$LIBBSD_ABI_MAJOR:$LIBBSD_ABI_MINOR:$LIBBSD_ABI_PATCH"
-AC_SUBST([LIBBSD_ABI])
+SOVERSION="$SOVERSION_MAJOR:$SOVERSION_MINOR:$SOVERSION_PATCH"
+AC_SUBST([SOVERSION])
 
 # Check and store if we got user supplied variables
 user_CFLAGS=${CFLAGS-unset}