Bump the -Wshadow cutoff to 4.8 rather than 4.6.

Per https://gcc.gnu.org/gcc-4.8/changes.html, -Wshadow was made less
trigger-happy starting 4.8.

Change-Id: I023919dea3497e2dd27679a6f0c4a114c0b30337
Reviewed-on: https://boringssl-review.googlesource.com/3730
Reviewed-by: Adam Langley <agl@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0a74166..7c7cca7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -60,7 +60,7 @@
   add_definitions(-DWIN32_LEAN_AND_MEAN)
 endif()
 
-if((CMAKE_COMPILER_IS_GNUCXX AND CMAKE_C_COMPILER_VERSION VERSION_GREATER "4.5.99") OR
+if((CMAKE_COMPILER_IS_GNUCXX AND CMAKE_C_COMPILER_VERSION VERSION_GREATER "4.7.99") OR
    CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wshadow")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wshadow")