Better preference for setting warnings level. See arduino/Arduino@61592d7#commitcomment-10668365

Author: Federico Fissore

platform.txt

@@ -1,5 +1,6 @@
 
 # Arduino SAM Core and platform.
+# ------------------------------
 #
 # For more info:
 # https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5---3rd-party-Hardware-specification
@@ -10,14 +11,19 @@ version=1.6.3
 # SAM3 compile variables
 # ----------------------
 
+compiler.warning_flags.none=-w
+compiler.warning_flags.normal=
+compiler.warning_flags.all=-Wall
+compiler.warning_flags.extra=-Wall -Wextra
+
 compiler.path={runtime.tools.arm-none-eabi-gcc.path}/bin/
 compiler.c.cmd=arm-none-eabi-gcc
-compiler.c.flags=-c -g -Os -w -ffunction-sections -fdata-sections -nostdlib --param max-inline-insns-single=500 -Dprintf=iprintf -MMD
+compiler.c.flags=-c -g -Os {compiler.warning_flags} -ffunction-sections -fdata-sections -nostdlib --param max-inline-insns-single=500 -Dprintf=iprintf -MMD
 compiler.c.elf.cmd=arm-none-eabi-gcc
 compiler.c.elf.flags=-Os -Wl,--gc-sections
 compiler.S.flags=-c -g -x assembler-with-cpp
 compiler.cpp.cmd=arm-none-eabi-g++
-compiler.cpp.flags=-c -g -Os -w -ffunction-sections -fdata-sections -nostdlib -fno-threadsafe-statics --param max-inline-insns-single=500 -fno-rtti -fno-exceptions -Dprintf=iprintf -MMD
+compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -ffunction-sections -fdata-sections -nostdlib -fno-threadsafe-statics --param max-inline-insns-single=500 -fno-rtti -fno-exceptions -Dprintf=iprintf -MMD
 compiler.ar.cmd=arm-none-eabi-ar
 compiler.ar.flags=rcs
 compiler.objcopy.cmd=arm-none-eabi-objcopy