[cmake] Temporarily revert enforcement of minimum GCC version increase

[tejohnson]

Summary

This is temporary, until bot that builds public facing LLVM
documentation is upgraded. It reverts only the cmake change in r284497,
but leaves the doc changes in place to preserve intent.

[aaron.ballman]

LGTM, thank you!

cmake/modules/CheckCompilerVersion.cmake

 # Check if the host compiler is new enough. LLVM requires at least GCC 4.8,
 # MSVC 2013, or Clang 3.1.
 
 include(CheckCXXSourceCompiles)
 
 if(NOT DEFINED LLVM_COMPILER_CHECKED)
   set(LLVM_COMPILER_CHECKED ON)
 
   if(NOT LLVM_FORCE_USE_OLD_TOOLCHAIN)
     if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
-      if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.8)
-        message(FATAL_ERROR "Host GCC version must be at least 4.8!")
+      # FIXME: Change this to 4.8 once documentation builder bot is upgraded
+      if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.7)
+        message(FATAL_ERROR "Host GCC version must be at least 4.7!")
       endif()
     elseif(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
       if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.1)
         message(FATAL_ERROR "Host Clang version must be at least 3.1!")
       endif()
 
       if (CMAKE_CXX_SIMULATE_ID MATCHES "MSVC")
         if (CMAKE_CXX_SIMULATE_VERSION VERSION_LESS 18.0)
elseif(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
         set(OLD_CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES})
         set(CMAKE_REQUIRED_FLAGS "-std=c++0x")
         check_cxx_source_compiles("
 #include <atomic>
 std::atomic<float> x(0.0f);
 int main() { return (float)x; }"
           LLVM_NO_OLD_LIBSTDCXX)
         if(NOT LLVM_NO_OLD_LIBSTDCXX)
-          message(FATAL_ERROR "Host Clang must be able to find libstdc++4.8 or newer!")
+          # FIXME: Change this to 4.8 once documentation builder bot is upgraded
+          message(FATAL_ERROR "Host Clang must be able to find libstdc++4.7 or newer!")
         endif()
         set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
         set(CMAKE_REQUIRED_LIBRARIES ${OLD_CMAKE_REQUIRED_LIBRARIES})
       endif()
     elseif(CMAKE_CXX_COMPILER_ID MATCHES "MSVC")
       if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 18.0)
         message(FATAL_ERROR "Host Visual Studio must be at least 2013")
       elseif(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 18.0.31101)
         message(WARNING "Host Visual Studio should at least be 2013 Update 4 (MSVC 18.0.31101)"
           "  due to miscompiles from earlier versions")
       endif()
     endif()
   endif()
 endif()