Adding Google benchmarking library.

third_party/google_benchmark/cmake/CXXFeatureCheck.cmake

+# - Compile and run code to check for C++ features
+#
+# This functions compiles a source file under the `cmake` folder
+# and adds the corresponding `HAVE_[FILENAME]` flag to the CMake
+# environment
+#
+#  cxx_feature_check(<FLAG> [<VARIANT>])
+#
+# - Example
+#
+# include(CXXFeatureCheck)
+# cxx_feature_check(STD_REGEX)
+# Requires CMake 2.8.12+
+
+if(__cxx_feature_check)
+  return()
+endif()
+set(__cxx_feature_check INCLUDED)
+
+function(cxx_feature_check FILE)
+  string(TOLOWER ${FILE} FILE)
+  string(TOUPPER ${FILE} VAR)
+  string(TOUPPER "HAVE_${VAR}" FEATURE)
+  if (DEFINED HAVE_${VAR})
+    return()
+  endif()
+  message("-- Performing Test ${FEATURE}")
+  try_run(RUN_${FEATURE} COMPILE_${FEATURE}
+          ${CMAKE_BINARY_DIR} ${CMAKE_CURRENT_SOURCE_DIR}/cmake/${FILE}.cpp
+          CMAKE_FLAGS ${BENCHMARK_CXX_LINKER_FLAGS}
+          LINK_LIBRARIES ${BENCHMARK_CXX_LIBRARIES})
+  if(RUN_${FEATURE} EQUAL 0)
+    message("-- Performing Test ${FEATURE} -- success")
+    set(HAVE_${VAR} 1 CACHE INTERNAL "Feature test for ${FILE}" PARENT_SCOPE)
+    add_definitions(-DHAVE_${VAR})
+  else()
+    if(NOT COMPILE_${FEATURE})
+      message("-- Performing Test ${FEATURE} -- failed to compile")
+    else()
+      message("-- Performing Test ${FEATURE} -- compiled but failed to run")
+    endif()
+  endif()
+endfunction()
+