A few changes to the build system:

• The list of modules is generated when running autogen.sh, not during configuring.
• The above process automatically adds various '--with-*' commands to enable/disable build of a module. They are added to the main configure script.
• Each module directory must have the .pexts_module file which contains one of the following words: enabled, disabled, default:
  
  enabled - the module is considered as a valid one, but it is not built by default (i.e. without explicitly passing --with-modulename) disabled - the module is considered to be invalid (not finished, not working etc.) and is never built. default - as 'enabled' but the module is built by default.
• The Makefile.in.pre files were removed and replaced with the Makefile.in files. The Makefile.in include src/Makefile.rules which is what used to be module_makefile.
• Cosmetics. Each module displays it's clearly separated name during the configure process.
• Cosmetics. After the configuration, a list of enabled modules is reported to the user.