meson: Change default of 'selinux' feature option to auto

There is really no reason for selinux to behave differently than other
options.

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/20231103211601.bgqx3cfq6pz2lmjw@awork3.anarazel.de

Author: anarazel

meson_options.txt

@@ -136,7 +136,7 @@ option('tcl_version', type: 'string', value: 'tcl',
 option('readline', type: 'feature', value: 'auto',
   description: 'Use GNU Readline or BSD Libedit for editing')
 
-option('selinux', type: 'feature', value: 'disabled',
+option('selinux', type: 'feature', value: 'auto',
   description: 'SELinux support')
 
 option('ssl', type: 'combo', choices: ['auto', 'none', 'openssl'],