Fix "No known features for CXX compiler", see
https://cmake.org/pipermail/cmake/2016-December/064733.html and the note at
https://cmake.org/cmake/help/v3.10/command/cmake_minimum_required.html
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -8,7 +8,7 @@
 #
 #========================================================================
 
-cmake_minimum_required(VERSION 2.8.8)
+cmake_minimum_required(VERSION 3.1.0)
 
 project(xpdf)