[LyX/master] Implement gcc version check for cmake

[Georg Baum]

commit c2433d8b8f2bd7be363ddc96bcb95116ac5ea8cd
Author: Georg Baum <b...@lyx.org>
Date:   Sun Jun 5 15:54:29 2016 +0200

    Implement gcc version check for cmake

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 33daf5b..68753c1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -260,11 +260,7 @@ if(LYX_ENABLE_CXX11 MATCHES "AUTO")
   else()
     if(CMAKE_CXX_COMPILER_ID MATCHES "GNU")
       execute_process(COMMAND ${CMAKE_CXX_COMPILER} -dumpversion 
OUTPUT_VARIABLE GCC_VERSION OUTPUT_STRIP_TRAILING_WHITESPACE)
-      if(NOT GCC_VERSION VERSION_LESS 4.3)
         set(LYX_ENABLE_CXX11 ON CACHE TYPE STRING FORCE)
-      else()
-        set(LYX_ENABLE_CXX11 OFF CACHE TYPE STRING FORCE)
-      endif()
     else()
       # Not a gnu compiler
       if(CMAKE_CXX_COMPILER_ID MATCHES "^[cC]lang$")
@@ -280,6 +276,9 @@ if(UNIX OR MINGW)
        execute_process(COMMAND ${CMAKE_CXX_COMPILER} -dumpversion 
OUTPUT_VARIABLE GCC_VERSION OUTPUT_STRIP_TRAILING_WHITESPACE)
        message(STATUS "Using GCC version ${GCC_VERSION}")
        if(GCC_VERSION VERSION_LESS 4.9)
+               if(GCC_VERSION VERSION_LESS 4.3)
+                       message(FATAL_ERROR "gcc >= 4.3 is required.")
+               endif()
                # <regex> in gcc is unusable in versions less than 4.9.0
                # see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53631
                set(LYX_USE_STD_REGEX 0)