build: make standalone builds work again

Apple's greendragon bot uses a standalone build of lldb which would
fail to build after a recent change to LLVM as it relied on LLVM setting
global flags for its build.  Attempt to repair the standalone build for
greendragon bot.

Author: compnerd

lldb/CMakeLists.txt

@@ -16,6 +16,10 @@ set(CMAKE_MODULE_PATH
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   project(lldb)
   include(LLDBStandalone)
+
+  set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform to")
+  set(CMAKE_CXX_STANDARD_REQUIRED YES)
+  set(CMAKE_CXX_EXTENSIONS NO)
 endif()
 
 include(LLDBConfig)