Build-system specifications for LibPSML 1.2

Here are a few proposals for LibPSML 1.2:

  - Remove support for the PSML_NO_OLD_API CPP option.
  - Rename m_interp to m_psml_interp.
  - Add a programming guideline to use positive statement for CPP options, instead of confusing double-negations, e.g. '#ifdef PSML_OLD_API' instead of '#ifndef PSML_NO_OLD_API'.
  - Start using the API version number for the library (see https://www.gnu.org/software/libtool/manual/html_node/Updating-version-info.html), starting with 1.0.0 and noting that it has *nothing* to do with the release number of the library.