Enable use of C++17 features

Resulting assembly listings of sampled example programs appears unchanged.  However, C++17 features make it easier to convince the compiler to put data into PROGMEM.

avr/platform.txt

@@ -19,12 +19,12 @@ compiler.warning_flags.all=-Wall -Wextra
 # Default "compiler.path" is correct, change only if you want to overidde the initial value
 compiler.path={runtime.tools.avr-gcc.path}/bin/
 compiler.c.cmd=avr-gcc
-compiler.c.flags=-c -g -Os {compiler.warning_flags} -std=gnu11 -ffunction-sections -fdata-sections -MMD {ltocflags}
+compiler.c.flags=-c -g -Os {compiler.warning_flags} -std=gnu17 -ffunction-sections -fdata-sections -MMD {ltocflags}
 compiler.c.elf.flags={compiler.warning_flags} -Os {ltoelfflags} -Wl,--gc-sections
 compiler.c.elf.cmd=avr-gcc
 compiler.S.flags=-c -g -x assembler-with-cpp {ltocppflags}
 compiler.cpp.cmd=avr-g++
-compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -MMD {ltocppflags}
+compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -std=c++17 -fpermissive -fno-sized-deallocation -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -MMD {ltocppflags}
 compiler.ar.cmd=avr-{ltoarcmd}ar
 compiler.ar.flags=rcs
 compiler.objcopy.cmd=avr-objcopy