Improve configure help layout
The result of ./configure --help
has a number of capitalization, quoting and spacing inconsistencies. Standard autoconf options have their descriptions start with lowercase, not all custom options do this. Quoting prevents parameters from being interpreted incorrectly and is not done everywhere. Finally, the AS_HELP_STRING macro should be used so spacing is automatically fixed and everything is aligned in neat columns. The attached patch fixes these inconsistencies.