Revert soname change

The soname is equal to current minus age.
In version 2.31.0 current is 2 and age is set to 0.
In version 2.31.1 current is 2 and age is set to 1.
This means the soname goes backwards from 2 to 1.
The full library version changes from 2.0.31 to 1.1.31

The soname should not go backwards, so this soname change looks like a
mistake that should be reverted.

The current, revision, age for a library should change in one of three ways:

1) increase current by one, reset revision and age to 0.
2) increase current by one, reset revision to 0 and increase age by 1.
3) increase revision by 1, retain the values of current and age.

1) is for non-backward compatible changes to the library (changes or
removals to the old ABI). Soname changes and applications using the
library must be recompiled.

2) is for when there are ABI additions to the library, but no ABI
changes or removals. Application compiled against the old version of
the library don't need to be recompiled, and the soname (current minus
age) does not change.

3) is for minor updates with no ABI additions, changes or removals.

The major, minor, patch version of the software project should not be
used as current, revision, age for the library. Especially true for
using the patch version as age, because that means the soname goes
backwards for patch releases as happened here.

Signed-off-by: Mattias Ellert <mattias.ellert@physics.uu.se>

Author: ellert

Makefile.am

@@ -53,7 +53,7 @@ include mem/Makefile.am
 # LIB version info not necessarily the same as package version
 LIBISAL_CURRENT=2
 LIBISAL_REVISION=31
-LIBISAL_AGE=1
+LIBISAL_AGE=0
 
 lib_LTLIBRARIES = libisal.la
 pkginclude_HEADERS += $(sort ${extern_hdrs})