Bump library soname

Also update maintainer documentation on binary compatibility testing.

README.maintainer

@@ -1,6 +1,16 @@
 Release Reminders
 =================
 
+ * These two commits broke binary compatibility:
+
+   a3576a73593987b26cd3eff346f8f7c11f713cbd
+   b8bdef0ad12883d72ced5eb443e6e34a93bbbb91
+
+   Need to figure out how. Every time any interface is added or any
+   object changes shape, always run compatibility tests by manually
+   running qtest-driver on most recent release using latest shared
+   library.
+
  * When making a release, always remember to run large file tests and
    image comparison tests (--enable-test-compare-images
    --with-large-file-test-path=/path).  For a major release, consider

TODO

@@ -46,18 +46,7 @@ Small, command-line tool only enhancements to do soon
    (libtool).
 
 
-Next ABI change
-===============
-
-Remove private methods that are there only for ABI compatibility
-including extra QPDFWriter writeTrailer, writeXRefTable,
-writeXRefStream.
-
-Remember that non-compatible changes to private methods still breaks
-ABI compatibility. See a3576a73593987b26cd3eff346f8f7c11f713cbd.
-
-
-5.3.0
+6.1.0
 =====
 
  * Add method to push inheritable resources to a single page by

libqpdf/build.mk

@@ -96,4 +96,4 @@ $(COBJS_libqpdf): libqpdf/$(OUTPUT_DIR)/%.$(LOBJ): libqpdf/%.c
 # * Otherwise, increment REVISION
 
 $(TARGETS_libqpdf): $(OBJS_libqpdf)
-	$(call makelib,$(OBJS_libqpdf),$@,$(LDFLAGS),$(LIBS),16,0,3)
+	$(call makelib,$(OBJS_libqpdf),$@,$(LDFLAGS),$(LIBS),17,0,0)