Make xorg deps optional

Fixes: #2

.SRCINFO

@@ -14,6 +14,9 @@ pkgbase = gdm
 	makedepends = plymouth
 	makedepends = python-packaging
 	makedepends = yelp-tools
+	makedepends = xorg-server
+	makedepends = xorg-xhost
+	makedepends = xorg-xrdb
 	depends = accountsservice
 	depends = audit
 	depends = bash
@@ -36,9 +39,9 @@ pkgbase = gdm
 	depends = systemd
 	depends = systemd-libs
 	depends = upower
-	depends = xorg-server
-	depends = xorg-xhost
-	depends = xorg-xrdb
+	optdepends = xorg-server: X session support
+	optdepends = xorg-xhost: X session support
+	optdepends = xorg-xrdb: X session support
 	source = git+https://gitlab.gnome.org/GNOME/gdm.git#commit=a5b591cd8d1db5c5d1ebe67d10ec3fe57b9bbded
 	source = 0001-Xsession-Don-t-start-ssh-agent-by-default.patch
 	b2sums = 23f2346014a2c7f39fb53627e1531d74eeb7636027e2192d8be72a1a553126796b0445b0c02691ee104835e333275881be77883b80252f9e0bc13a37a152fe6b
@@ -69,9 +72,6 @@ pkgname = gdm
 	depends = systemd
 	depends = systemd-libs
 	depends = upower
-	depends = xorg-server
-	depends = xorg-xhost
-	depends = xorg-xrdb
 	depends = libgdm
 	optdepends = fprintd: fingerprint authentication
 	backup = etc/gdm/PostSession/Default

PKGBUILD

@@ -36,9 +36,6 @@ depends=(
   systemd
   systemd-libs
   upower
-  xorg-server
-  xorg-xhost
-  xorg-xrdb
 )
 makedepends=(
   dconf
@@ -49,6 +46,14 @@ makedepends=(
   plymouth
   python-packaging
   yelp-tools
+  xorg-server
+  xorg-xhost
+  xorg-xrdb
+)
+optdepends=(
+  'xorg-server: X session support'
+  'xorg-xhost: X session support'
+  'xorg-xrdb: X session support'
 )
 checkdepends=(check)
 _commit=a5b591cd8d1db5c5d1ebe67d10ec3fe57b9bbded  # tags/46.0^0