Adding Google benchmarking library.

third_party/google_benchmark/cmake/GetGitVersion.cmake

+# - Returns a version string from Git tags
+#
+# This function inspects the annotated git tags for the project and returns a string
+# into a CMake variable
+#
+#  get_git_version(<var>)
+#
+# - Example
+#
+# include(GetGitVersion)
+# get_git_version(GIT_VERSION)
+#
+# Requires CMake 2.8.11+
+find_package(Git)
+
+if(__get_git_version)
+  return()
+endif()
+set(__get_git_version INCLUDED)
+
+function(get_git_version var)
+  if(GIT_EXECUTABLE)
+      execute_process(COMMAND ${GIT_EXECUTABLE} describe --match "v[0-9]*.[0-9]*.[0-9]*" --abbrev=8
+          RESULT_VARIABLE status
+          OUTPUT_VARIABLE GIT_VERSION
+          ERROR_QUIET)
+      if(${status})
+          set(GIT_VERSION "v0.0.0")
+      else()
+          string(STRIP ${GIT_VERSION} GIT_VERSION)
+          string(REGEX REPLACE "-[0-9]+-g" "-" GIT_VERSION ${GIT_VERSION})
+      endif()
+
+      # Work out if the repository is dirty
+      execute_process(COMMAND ${GIT_EXECUTABLE} update-index -q --refresh
+          OUTPUT_QUIET
+          ERROR_QUIET)
+      execute_process(COMMAND ${GIT_EXECUTABLE} diff-index --name-only HEAD --
+          OUTPUT_VARIABLE GIT_DIFF_INDEX
+          ERROR_QUIET)
+      string(COMPARE NOTEQUAL "${GIT_DIFF_INDEX}" "" GIT_DIRTY)
+      if (${GIT_DIRTY})
+          set(GIT_VERSION "${GIT_VERSION}-dirty")
+      endif()
+  else()
+      set(GIT_VERSION "v0.0.0")
+  endif()
+
+  message("-- git Version: ${GIT_VERSION}")
+  set(${var} ${GIT_VERSION} PARENT_SCOPE)
+endfunction()