config: close a loophole where user input means x server has a feature
config: close a loophole where user input means x server has a feature

If a user provides a path in --with-xorg-conf-dir,
the configuartion assumes the server does have a config path which may
not be the case.

Resolve this by testing the value of the server returned sysconfigdir
rather than the user provided value.

Signed-off-by: Gaetan Nadon <8a8cc541338200f67d4d0bcb65c78b6166649462@videotron.ca>

quadpixels/three-finger-dragging,nanderson94/xf86-input-synaptics,quadpixels/three-finger-dragging,ssaavedra/xf86-input-synaptics,sencer/synaptics,corcoran/xserver-xorg-input-synaptics,joeshaw/xf86-input-synaptics,eyko/xf86-input-synaptics,corcoran/xserver-xorg-input-synaptics,jiixyj/xf86-input-synaptics,philipn/xserver-xorg-input-synaptics,RsrchBoy/xserver-xorg-input-synaptics,kelnos/xf86-input-synaptics,mortehu/xf86-input-synaptics,felipejfc/xserver-xorg-input-synaptics,mmonaco/xf86-input-synaptics,RsrchBoy/xserver-xorg-input-synaptics,eyko/xf86-input-synaptics,sencer/synaptics,ssaavedra/xf86-input-synaptics,felipejfc/xserver-xorg-input-synaptics,gdestuynder/xf86-input-synaptics,felipejfc/xserver-xorg-input-synaptics,Lyude/xf86-input-synaptics,Lyude/xf86-input-synaptics,nanderson94/xf86-input-synaptics,joeshaw/xf86-input-synaptics,philipn/xserver-xorg-input-synaptics,jiixyj/xf86-input-synaptics,kelnos/xf86-input-synaptics,mmonaco/xf86-input-synaptics,gdestuynder/xf86-input-synaptics,mortehu/xf86-input-synaptics,philipn/xserver-xorg-input-synaptics