Debian must set SOVERSION used by Makefile.
This is a misconception. It will be fixed in next upstream.
Index: b/Makefile
===================================================================
--- a/Makefile	2010-01-21 01:50:28.000000000 +0100
+++ b/Makefile	2010-01-21 01:51:02.000000000 +0100
@@ -24,7 +24,7 @@
 # USA
 
 export LIB_NAME := texttools
-SOVERSION := 1
+SOVERSION ?= 1
 export SONAME := lib$(LIB_NAME).so.$(SOVERSION)
 export LIB_DIR := .
 
