Explicitly look for X11 when using it.

This fixes the build with CMake 3.2, whose FindOpenGL.cmake stopped
calling find_package(X11).

Instead, do that ourselves when Q_WS_X11 is true, since that is when we
already try to link against libX11.

Sent upstream: https://github.com/cryos/avogadro/pull/13
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -230,6 +230,10 @@ if(NOT Linguist_FOUND)
   message(WARNING " Qt4 Linguist not found, please install it if you want Avogadro translations")
 endif()
 
+if(Q_WS_X11)
+  find_package(X11 REQUIRED) # avogadro/src/main.cpp calls XInitThread().
+endif()
+
 find_package(Eigen3) # find and setup Eigen3 if available
 if(NOT EIGEN3_FOUND)
    message(STATUS "Cannot find Eigen3, trying Eigen2")