Fix quoting of xfb-run invocation

Works For Me (Ubuntu 14.04.2)

CMakeLists.txt

@@ -293,7 +293,7 @@ IF (ENABLE_TESTS)
     IMMEDIATE @ONLY)
   # For server side testing we have no X, we can use xfvb as a fake x
   # sudo apt-get install xfvb
-  add_custom_target(check-no-x COMMAND xvfb-run --server-args="-screen 10 1024x768x24" ctest --output-on-failure)
+  add_custom_target(check-no-x COMMAND xvfb-run --server-args=-screen\ 10\ 1024x768x24 ctest --output-on-failure)
 ENDIF (ENABLE_TESTS)
 
 IF (SUPPRESS_QT_WARNINGS)