Allow build with other than C++11 standard. (#392)

Don't override CMAKE_CXX_STANDARD if it's already set.

Author: Dmitry

CMakeLists.txt

@@ -25,8 +25,10 @@ project(opencensus-cpp VERSION 0.5.0 LANGUAGES CXX)
 
 option(FUZZER "Either OFF or e.g. -fsanitize=fuzzer,address" OFF)
 
-set(CMAKE_CXX_STANDARD 11)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
+if(NOT CMAKE_CXX_STANDARD)
+  set(CMAKE_CXX_STANDARD 11)
+  set(CMAKE_CXX_STANDARD_REQUIRED ON)
+endif()
 
 include(CTest) # Defines option BUILD_TESTING.
 enable_testing()