configure.ac: fix SYSCONFDEFDIR default value

configure.ac: fix SYSCONFDEFDIR default value

configure.ac was using AC_ARG_WITH incorrectly, yielding a generated configure script like:

    # Check whether --with-sysconfdefdir was given.
    if test "${with_sysconfdefdir+set}" = set; then :
      withval=$with_sysconfdefdir; SYSCONFDEFDIR=$withval
    else
      "/etc/sysconfig"
    fi

This commit fixes the default case so that the SYSCONFDEFDIR variable is assigned the value "/etc/sysconfig" rather than trying to execute "/etc/sysconfig" as a command.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>