Stop using -release, reset abi version fields

The use of `-release` forces recompilation of reverse dependencies
on every new release and makes the abi version fields essentially
cosmetic (even if they correctly track (in)compatibility).

Clarified the comments on how to handle the abi fields on releases,
since the short forms didn't cover resetting the age/revision flags.

Reordered the field declarations in the automake file to match the
order in which they are passed to -version-info.

Makefile.am

@@ -90,7 +90,6 @@ LIBS = $(JSON_LIBS) $(GLIB_LIBS) @LIBS@
 lib_LTLIBRARIES = libmypaint.la
 
 libmypaint_la_LDFLAGS = \
-    -release @LIBMYPAINT_API_PLATFORM_VERSION@ \
     -version-info @LIBMYPAINT_ABI_VERSION_INFO@ \
     -no-undefined
 

configure.ac

@@ -17,9 +17,21 @@ m4_define([libmypaint_api_prerelease], [])  # may be blank
 # https://www.gnu.org/software/libtool/manual/html_node/Updating-version-info.html
 # The rules are fiddly, and are summarized here.
 
-m4_define([libmypaint_abi_revision], [2])  # increment on every release
-m4_define([libmypaint_abi_current], [2])  # inc when add/remove/change interfaces
-m4_define([libmypaint_abi_age], [1])  # inc only if changes backward compat
+# NOTE: Due to the use of the ``-release`` flag, prior to the 1.6.1 release,
+# the ABI version fields were not used in a meaningful way, since the SONAME
+# differed even for ABI-compatible versions of the library.
+
+# Increment if any public interfaces have been
+# added/removed/changed since the last release.
+m4_define([libmypaint_abi_current], [0])
+
+# Set to 0 if ``current`` is incremented, otherwise increment
+# if the __code__ has changed at all since the last release.
+m4_define([libmypaint_abi_revision], [0])
+
+# Set to 0 if any public interfaces have been removed/changed since the last release,
+# otherwise increment if any public interfaces have been __added__.
+m4_define([libmypaint_abi_age], [0])  # inc only if changes backward compat
 
 
 ## Derivative version macros ##