ltoptions.m4: Make aix-soname assignment more clear

ltoptions.m4: Make aix-soname assignment more clear

The option --with-aix-soname=svr4 does not work the same as using
LT_INIT([aix-soname=svr4]), since the configure script is reordered
depending on which is used. The reordering causes 'enable_shared'
to not be set before processing for 'aix-soname' when using LT_INIT.

For LT_INIT([aix-soname=svr4]) to be set as intended, 'shared' must
be set before 'aix-soname=svr4', like LT_INIT([shared aix-soname=svr4]).

* m4/ltoptions.m4: Warn user if 'enable_shared' has not been set when
  setting 'aix-soname' and add configuration messages to show which
  variant of shared library versioning has been set for 'aix-soname'.
* doc/libtool.texi: Update to specify need for 'shared' to be used
  before 'aix-soname' for LT_INIT.