Move to -std=g++11 from -std=c++11 (#6339)

Looking at other projects, they're all building using gnu extensions
(-std=gnu++11).  We're already embedding gnu-specific pragmas for
compile options, so I see no reason not to fall in line w/others on
this.

Fixes #4639

Author: earlephilhower

platform.txt

@@ -34,6 +34,7 @@ build.sslflags=
 
 build.exception_flags=-fno-exceptions
 build.stdcpp_lib=-lstdc++
+build.stdcpp_level=-std=gnu++11
 
 build.float=-u _printf_float -u _scanf_float
 build.led=
@@ -57,7 +58,7 @@ compiler.c.elf.cmd=xtensa-lx106-elf-gcc
 compiler.c.elf.libs=-lhal -lphy -lpp -lnet80211 {build.lwip_lib} -lwpa -lcrypto -lmain -lwps -lbearssl -laxtls -lespnow -lsmartconfig -lairkiss -lwpa2 {build.stdcpp_lib} -lm -lc -lgcc
 
 compiler.cpp.cmd=xtensa-lx106-elf-g++
-compiler.cpp.flags=-c {compiler.warning_flags} -Os -g -mlongcalls -mtext-section-literals -fno-rtti -falign-functions=4 -std=c++11 -MMD -ffunction-sections -fdata-sections {build.exception_flags} {build.sslflags}
+compiler.cpp.flags=-c {compiler.warning_flags} -Os -g -mlongcalls -mtext-section-literals -fno-rtti -falign-functions=4 {build.stdcpp_level} -MMD -ffunction-sections -fdata-sections {build.exception_flags} {build.sslflags}
 
 compiler.as.cmd=xtensa-lx106-elf-as