Add missing build flags.

I would like to use C++17.

platformio.ini

@@ -25,6 +25,8 @@ include_dir = src
 ;filter for source file directory when building
 ; see https://docs.platformio.org/en/stable/projectconf/sections/env/options/build/build_src_filter.html
 build_src_filter = +<*> -<.git/> -<.svn/>
+build_flags = -std=gnu++17 -Werror=return-type
+build_unflags = -std=gnu++11
 
 ;general dependencies
 ; for version requirements see https://docs.platformio.org/en/stable/core/userguide/pkg/cmd_install.html#cmd-pkg-install-requirements