comics enable/disable in configure.ac

[Jani Monoses <jani monoses gmail com>]

Hello
configure.ac seems to disregard whether enable or disable is given for comics
This change makes it be like the other backend checks.

 AC_ARG_ENABLE(comics,
-       [AC_HELP_STRING([--enable-comics], [Compile with support for comic book archives])],enable_comics=yes,enable_comics=no)
+       [AC_HELP_STRING([--enable-comics], [Compile with support for comic book archives])],enable_comics="$enableval",enable_comics=no)