Fixed ioqueue backend selection configure script

[sauwming]

To fix #3877

https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Package-Options.html

AC_ARG_ENABLE (feature, help-string, [action-if-given], [action-if-not-given])
     --enable-feature[=arg]
     --disable-feature
If the user gave configure the option --enable-feature or --disable-feature, run shell commands
action-if-given.
If neither option was given, run shell commands action-if-not-given.

The user can give an argument by following the feature name with ‘=’ and the argument.
Giving an argument of ‘no’  requests that the feature not be made available. If no argument is given,
it defaults to ‘yes’. --disable-feature is equivalent to --enable-feature=no.