Make the port build with CMake 3.4.0.

Stop relying on an implicit inclusion of the CheckIncludesFile module.
Not upstreamable, the latest upstream version does not use that module
anymore.
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -3,6 +3,7 @@ project(kgamma)
 find_package(KDE4 REQUIRED)
 include(KDE4Defaults)
 include(MacroLibrary)
+include(CheckIncludeFiles)
 
 find_package(X11 REQUIRED)