[Rework] Require proper C++ environment for Rspamd build

CMakeLists.txt

 
 # This is supported merely with cmake 3.1
 SET(CMAKE_C_STANDARD 11)
+SET(CMAKE_CXX_STANDARD 17)
 SET(CMAKE_C_STANDARD_REQUIRED ON)
+SET(CMAKE_CXX_STANDARD_REQUIRED ON)
 LIST(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/")
 
 SET(RSPAMD_MASTER_SITE_URL "https://rspamd.com")

src/CMakeLists.txt

 		FOREACH(_lib ${CLANG_EXTRA_PLUGINS_LIBS})
 			SET(CMAKE_C_FLAGS
 					"${CMAKE_C_FLAGS} -Xclang -load -Xclang ${_lib}")
+			SET(CMAKE_CXX_FLAGS
+					"${CMAKE_CXX_FLAGS} -Xclang -load -Xclang ${_lib}")
 		ENDFOREACH()
 	ENDIF()
 	IF(CLANG_EXTRA_PLUGINS)
 		FOREACH(_plug ${CLANG_EXTRA_PLUGINS})
 			SET(CMAKE_C_FLAGS
 					"${CMAKE_C_FLAGS} -Xclang -add-plugin -Xclang ${_plug}")
+			SET(CMAKE_CXX_FLAGS
+					"${CMAKE_C_FLAGS} -Xclang -add-plugin -Xclang ${_plug}")
 		ENDFOREACH()
 	ENDIF()
 ENDIF ()