Keep -std=gnu++11 in stm32 build_unflags

platformio.ini

@@ -380,7 +380,7 @@ src_filter    = ${common.default_src_filter} +<src/HAL/STM32>
 platform      = ${common_stm32.platform}
 build_flags   = !python Marlin/src/HAL/STM32F1/build_flags.py
   ${common.build_flags} -std=gnu++14 -DHAVE_SW_SERIAL
-build_unflags = -std=gnu11
+build_unflags = -std=gnu11 -std=gnu++11
 src_filter    = ${common.default_src_filter} +<src/HAL/STM32F1>
 lib_ignore    = SPI
 lib_deps      = ${common.lib_deps}