diff --git a/var/spack/repos/builtin/packages/qt/package.py b/var/spack/repos/builtin/packages/qt/package.py
index 7e1d9e27583..e61b759d48c 100644
--- a/var/spack/repos/builtin/packages/qt/package.py
+++ b/var/spack/repos/builtin/packages/qt/package.py
@@ -125,15 +125,12 @@ class Qt(Package):
     # QtQml
     depends_on("python", when='@5.7.0:', type='build')
 
-    # OpenGL hardware acceleration
-    depends_on("mesa", when='@4:+opengl')
-
-    depends_on("libxcb", when=sys.platform != 'darwin')
-    depends_on("libx11", when=sys.platform != 'darwin')
-
-    # This is only needed for qt@3:4.99, but combining
-    # spec string with boolean expression does not work:
-    depends_on("libxext", when=sys.platform != 'darwin')
+    # X window system:
+    if sys.platform != 'darwin':
+        depends_on("mesa", when='@4:+opengl')
+        depends_on("libxcb")
+        depends_on("libx11")
+        depends_on("libxext", when='@3:4.99')
 
     # Webkit
     depends_on("flex", when='+webkit', type='build')