Use here-document for 'configure --help' output. (#714)

Details:
- Changed the configure script function that outputs "--help" text to do 
  so via so-called "here-document" syntax for improved readability and
  maintainability. The change eliminates hundreds of echo statements and 
  makes it easier to change existing configure options' help text, along
  with other benefits such as eliminating the need to escape double-
  quote characters (").