build: set cxx standard to 17 in dev mode

CMakeLists.txt

@@ -16,6 +16,14 @@ option(WITH_DESKTOP_ENTRY "Ask to install desktop entry on first startup" ON)
 option(WITH_UPDATER "Regularly check for new updates" ON)
 option(DEV_MODE "Checkout latest monero master on build" OFF)
 
+if(DEV_MODE)
+  # DEV_MODE checks out the monero submodule to master, which requires C++17.
+  set(CMAKE_CXX_STANDARD 17)
+else()
+  set(CMAKE_CXX_STANDARD 11)
+endif()
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
 list(INSERT CMAKE_MODULE_PATH 0 "${CMAKE_SOURCE_DIR}/cmake")
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
@@ -442,7 +450,7 @@ message(STATUS "Using C++ security hardening flags: ${CXX_SECURITY_FLAGS}")
 message(STATUS "Using linker security hardening flags: ${LD_SECURITY_FLAGS}")
 
 set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c11 ${C_SECURITY_FLAGS}")
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 ${CXX_SECURITY_FLAGS}")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${CXX_SECURITY_FLAGS}")
 set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${LD_SECURITY_FLAGS} ${STATIC_FLAGS}")
 
 add_subdirectory(translations)