cmake: add -libstd=libc++ for linux clang build

This is needed to match the compile flags of the Unreal Engine

cmake/cmake-modules/CommonSetup.cmake

@@ -1,6 +1,9 @@
 # Common setup instructions shared by all AirSim CMakeLists.
 
 macro(CommonSetup)
+
+    find_package(Threads REQUIRED)
+
     message(STATUS "Running CommonSetup...")   
 
     find_path(AIRSIM_ROOT NAMES AirSim.sln PATHS ".." "../.." "../../.." "../../../.." "../../../../.." "../../../../../..")
@@ -21,7 +24,13 @@ macro(CommonSetup)
         else ()
 		    ##TODO: Werror removed temporarily. It should be added back after Linux build is stable
             set(CMAKE_CXX_FLAGS "-std=c++14 -ggdb -lpthread -Wall -Wextra -Wstrict-aliasing -fmax-errors=2 -Wunreachable-code -Wcast-qual -Wctor-dtor-privacy -Wdisabled-optimization -Wformat=2 -Winit-self -Wmissing-include-dirs -Wnoexcept -Wold-style-cast -Woverloaded-virtual -Wredundant-decls -Wshadow -Wstrict-null-sentinel -Wstrict-overflow=5 -Wswitch-default -Wundef -Wno-unused -Wno-variadic-macros -Wno-parentheses -fdiagnostics-show-option ${MAVLINK_OVERRIDES} ${BOOST_OVERRIDES} ${RPC_LIB_DEFINES} -ldl ${CMAKE_CXX_FLAGS}")
-            set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ")
+
+            if (${CMAKE_CXX_COMPILER_ID} MATCHES "Clang")
+                # make sure to match the compiler flags with which the Unreal
+                # Engine is built with
+                set(CMAKE_CXX_FLAGS "-stdlib=libc++ ${CMAKE_CXX_FLAGS}")
+                set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -stdlib=libc++ -lc++ -lc++abi")
+            endif ()
         endif ()
         set(BUILD_PLATFORM "x64")
         set(CMAKE_POSITION_INDEPENDENT_CODE ON)
@@ -31,8 +40,6 @@ macro(CommonSetup)
         set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} /NXCOMPAT /DYNAMICBASE /INCREMENTAL:NO ")
     ENDIF()
 
-    find_package(Threads REQUIRED)
-
     ## common boost settings to make sure we are all on the same page
     set(Boost_USE_STATIC_LIBS ON) 
     set(Boost_USE_MULTITHREADED ON)