Skip XRANDR check if X11 is not found.

Our code does not compile when HAVE_XRANDR=1 and HAVE_X11=0.
This combination would not normally exist, but on OS X we skip the X11
check so it is possible for X11 to be "not found" even when the OS does
support it. On OS X 10.6 this was not a problem because XRANDR is not
detected there, but apparently on OS X 10.7 XRANDR is available.

CMakeLists.txt

@@ -287,7 +287,9 @@ else()
 	add_definitions(-DHAVE_X11=0)
 endif()
 
+if(X11_FOUND)
 check_lib(XRANDR Xrandr)
+endif()
 if(XRANDR_FOUND)
 	add_definitions(-DHAVE_XRANDR=1)
 else()