CMake: check CMAKE_CXX_COMPILER_ID, not CMAKE_C_COMPILER_ID

This may or may not have gotten broken now that benchmark was marked as CXX-only.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 41b3b66..9db1361 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -191,7 +191,7 @@
   # Link time optimisation
   if (BENCHMARK_ENABLE_LTO)
     add_cxx_compiler_flag(-flto)
-    if ("${CMAKE_C_COMPILER_ID}" STREQUAL "GNU")
+    if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
       find_program(GCC_AR gcc-ar)
       if (GCC_AR)
         set(CMAKE_AR ${GCC_AR})
@@ -200,7 +200,7 @@
       if (GCC_RANLIB)
         set(CMAKE_RANLIB ${GCC_RANLIB})
       endif()
-    elseif("${CMAKE_C_COMPILER_ID}" MATCHES "Clang")
+    elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
       include(llvm-toolchain)
     endif()
   endif()