Suppress Clang warning for unsupported warning.

Clang doesn't support the unused-lambda-capture warning until version
5.0: http://releases.llvm.org/5.0.0/tools/clang/docs/DiagnosticsReference.html#wunused-lambda-capture

Bug b/123933266

Change-Id: I6c5681a64e08ab479d726679ec18447dec926630
Reviewed-on: https://swiftshader-review.googlesource.com/c/24448
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 054ad5b..7da2b52 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -205,7 +205,8 @@
     set_cpp_flag("-Werror=missing-braces")
     set_cpp_flag("-fno-exceptions")
 
-    if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+    if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND
+       CLANG_VERSION_STRING VERSION_GREATER_EQUAL 5)
         set_cpp_flag("-Werror=unused-lambda-capture")
     endif()