Print error message if trying to do a native Unix build

CMakeLists.txt

@@ -7,7 +7,7 @@ cmake_minimum_required(VERSION 2.6)
 project(libjpeg-turbo)
 set(VERSION 1.0.80)
 
-if(UNIX OR MINGW OR CYGWIN)
+if(MINGW OR CYGWIN)
   execute_process(COMMAND "date" "+%Y%m%d" OUTPUT_VARIABLE BUILD)
   string(REGEX REPLACE "\n" "" BUILD ${BUILD})
 elseif(WIN32)
@@ -15,7 +15,7 @@ elseif(WIN32)
     OUTPUT_VARIABLE BUILD)
   string(REGEX REPLACE "\n" "" BUILD ${BUILD})
 else()
-  message(FATAL_ERROR "date not implemented")
+  message(FATAL_ERROR "Platform not supported by this build system.  Use autotools instead.")
 endif()
 
 if(NOT CMAKE_BUILD_TYPE)