configure.ac: Adding version string for LIB_NGSPICE_API

This isn't currently used but can be used in various places to
automatically provide the internal API version that is used.
Normally developers doesn't want to break the API by introducing a
version bump as this means the new library isn't backward compatible
any more.

E.g. the API version can be used to enhance the name of the library,
quite often libraries with an API version 0 don't add this version to
library name (like ngspice is doing this too right now).

configure.ac

@@ -127,6 +127,10 @@ AC_SUBST(LT_NGSPICE_REVISION)
 AC_SUBST(LT_NGSPICE_AGE)
 AC_SUBST(LIBNGSPICE_SO_VERSION)
 
+# Defining the API Version
+LIBNGSPICE_API_VERSION=0
+AC_SUBST(LIBNGSPICE_API_VERSION)
+
 # Package Options
 # ---------------