libtool: Adding helping content for API versioning

Doing the libtool versioning right can be a bit tedious. Especially the
correct handling of the various versions for the libtool .so naming is
difficult to understand if this part isn't done often.

To improve the API versioning adding some helping text what the various
version number are intended for and how to handle these within the
life cycle and development of the Ngspice project.

Basically libtool uses 'current', 'revision' and 'age' to version a
library.
Here a short outline how to do the libtool versioning:

 Only bug fixing did happen, no new symbols were added, the new library
 provide the exact amount and behaviour of the existing symbols.

 --> Increase the 'revision' by 1 (c:r+1:a)

 New symbols were added, the behaviour of the existing symbols doesn't
 have changed.

 --> Increase the 'age' by 1, set 'revision' to 0 (c:r=0,a+1)

 Existing symbols behave now different or existing symbols were removed.

 --> Increase 'current' by 1, set 'revison' to 0, set 'age' to 0 (c+1,r=0,a=0)
 !!!Note!!!
 The ABI version is also affected by this (needs a bump too, the library
 isn't backward compatible any more.

configure.ac

@@ -27,9 +27,6 @@ AC_INIT([ngspice], [ngspice_version], [http://ngspice.sourceforge.net/bugrep.htm
 # Revision stamp the generated ./configure script
 AC_REVISION([$Revision: ngspice_version$])
 
-# Libtool shared ngspice versioning info
-AC_SUBST([NG_SO_VERSION], [0:1:0])
-
 # Unique file in the source directory
 AC_CONFIG_SRCDIR([src/ngspice.c])
 
@@ -52,7 +49,83 @@ AC_SUBST([AM_CPPFLAGS], ['-I. -I$(srcdir) -I$(top_builddir)/src/include'])
 
 AC_CONFIG_MACRO_DIR([m4])
 
+# Enable the automatically build of shared and static libraries
+LT_INIT([shared static])
 
+# Setting the libtool versioning
+###################################################################################
+#                                                                                 #
+#  To set the version of the library, libtool provides the -version-info          #
+#  parameter, which accepts three numbers, separated by colons, that are called   #
+#  respectively, current, revision and age. Both their name and their behavior,   #
+#  nowadays, have to be considered fully arbitrary, as the explanation provided   #
+#  in the official documentation is confusing to say the least, and can be, in    #
+#  some cases, considered completely wrong.                                       #
+#  https://autotools.io/libtool/version.html                                      #
+#                                                                                 #
+###################################################################################
+#
+# How to work with the libtool versioning?
+#
+# Follow the following steps from top to bottom. This means always start at point 1
+# if you plan to make a release and change the values.
+# Every new library starts with a version 'current'  (short 'c') = 0
+#                                         'revision' (short 'r') = 0
+#                                         'age'      (short 'a') = 0
+#
+# Update the libtool versioning only after the release of a public release of ngspice.
+# Go through the following checklist from top to bottom and check your needs, following
+# the reminded changes if you can say "Yes" for specific check.
+#
+# 1. Only existing code has changed, no functional changes.
+#    If the library source code has changed but *no* new symbols were added at all
+#    since the last update, then increment the revision (c:r:a becomes c:r+1:a).
+#    This is usually happen if the existing source of a function was changed for
+#    bug fixing e.g.
+#
+#  --> Increase the 'LT_NGSPICE_REVISION' value with *every* new software release
+#      within one release cycle.
+#
+# 2. Interfaces were added, functions have changed or are removed.
+#    If any interfaces [exported functions or data] have been added, got internal
+#    changes that implies a different behavior or removed and by this the visible
+#    symbols have changed since the last update, increment current, and set the
+#    revision to 0 (c:r:a becomes c+1:r=0:a).
+#    The new modified behavior of the library isn't backward compatible!
+#
+#  --> Increase the 'LT_NGSPICE_CURRENT' value whenever as an interface has been added
+#      or removed. This implies also a API change! You mostly have to change the
+#      'ngspice_major_version'!
+#  --> Set 'LT_NGSPICE_REVISION' to 0.
+#
+# 3. Interfaces were added but none removed or changed.
+#    If any interfaces have been added since the last public release and non of the
+#    existing interfaces were removed and existing interfaces have not changed internal
+#    functionality then the new library is backward compatible. Existing binaries can
+#    use the new library the same way than as the existing old library without loosing
+#    existing functionality or breakage.
+#    Increase age by 1 (c:r:a becomes c:r:a+1).
+#
+#  --> Increase the 'LT_NGSPICE_AGE' value only if the changes made to the ABI are
+#      backward compatible.
+#
+# 4. Interfaces were removed or have functional changes.
+#    If any interfaces within the library have been removed since the last public
+#    release or got some internal changes that let the interface act different than
+#    before, then set age to 0. The library isn't backwards compatible.
+#
+#  --> Set 'LT_NGSPICE_AGE' to 0.
+
+LT_NGSPICE_CURRENT=0
+LT_NGSPICE_REVISION=1
+LT_NGSPICE_AGE=0
+LIBNGSPICE_SO_VERSION=$LT_NGSPICE_CURRENT.$LT_NGSPICE_REVISION.$LT_NGSPICE_AGE
+
+# Announce the libtool version
+AC_SUBST(LT_NGSPICE_CURRENT)
+AC_SUBST(LT_NGSPICE_REVISION)
+AC_SUBST(LT_NGSPICE_AGE)
+AC_SUBST(LIBNGSPICE_SO_VERSION)
 
 # Package Options
 # ---------------