title: Cmake 'check_include_files' function failed when files exist
body: <p>Assume we have a CMakeLists.txt who has code like ( following is little part of whole file ):</p>
  
  <pre><code>cmake_minimum_required(VERSION 2.6.3)
  #...
  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -I/usr/include/mit-krb5/")
  message("CMAKE_REQUIRED_INCLUDES=${CMAKE_REQUIRED_INCLUDES};CMAKE_REQUIRED_DEFINITIONS=${CMAKE_REQUIRED_DEFINITIONS};CMAKE_REQUIRED_FLAGS=${CMAKE_REQUIRED_FLAGS}")
  message("before find path, fuckGssApiIncludes=${fuckGssApiIncludes}")
  find_path(fuckGssApiIncludes "gssapi.h" "/usr/include")
  if(NOT fuckGssApiIncludes)
     message(FATAL_ERROR "Can't find folder containing gssapi.h")
  endif()
  message("after, fuckGssApiIncludes=${fuckGssApiIncludes}")
  include_directories(${fuckGssApiIncludes})
  check_include_files("gssapi.h" HAVE_GSSAPI_H)
  #....
  if (NOT HAVE_GSSAPI_H)
    message(FATAL_ERROR "Cannot find GSS libraries")
  </code></pre>
  
  <p>And we have some files:</p>
  
  <pre><code># ls /usr/include/gssapi.h
  /usr/include/gssapi.h
  </code></pre>
  
  <p>But when I run this command with <strong>cmake version 3.12.1</strong>, I got error like this:</p>
  
  <pre><code>CMAKE_REQUIRED_INCLUDES=/usr/include/;/usr/include/x86_64-linux-gnu/;CMAKE_REQUIRED_DEFINITIONS=;CMAKE_REQUIRED_FLAGS=
  before find path, fuckGssApiIncludes=
  after, fuckGssApiIncludes=/usr/include
  CMake Error at CMakeLists.txt:548 (message):
    Cannot find GSS libraries
  </code></pre>
  
  <p>I thought in someway, Cmake.check_include_files did not works as Cmake's definition.</p>
  
  <p>So, <strong>Q1: Is my thought correct, and why?</strong>  <strong>Q2: How to work around, and make this cmakelists.txt run well?</strong></p>