Added -fpermissive flag.

This avoid build errors on old libraries. It may possibly be removed in
the future after a period of transition.

platform.txt

@@ -25,7 +25,7 @@ compiler.c.elf.flags={compiler.warning_flags} -Os -Wl,--gc-sections
 compiler.c.elf.cmd=avr-gcc
 compiler.S.flags=-c -g -x assembler-with-cpp
 compiler.cpp.cmd=avr-g++
-compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -std=gnu++11 -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -MMD
+compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -MMD
 compiler.ar.cmd=avr-ar
 compiler.ar.flags=rcs
 compiler.objcopy.cmd=avr-objcopy