config: comment, minor upgrade, quote and layout configure.ac

Group statements per section as per Autoconf standard layout
Quote statements where appropriate.
Autoconf recommends not using dnl instead of # for comments

Use AC_CONFIG_FILES to replace the deprecated AC_OUTPUT with parameters.

This helps automated maintenance and release activities.
Details can be found in http://wiki.x.org/wiki/NewModuleGuidelines



Signed-off-by: Gaetan Nadon <memsize@videotron.ca>