Force -fno-common on old GCC versions

Force older GCC versions to place uninitialized global variables in
the BSS section. This will cause linking errors if the same global
variable is defined in different compilation units that should be
linked in the end.

Author: augustofg

toolchain/toolchain-arm-none-eabi.cmake

@@ -26,7 +26,7 @@ set( CMAKE_OBJCOPY ${TC_PATH}${CROSS_COMPILE}objcopy
 set( CMAKE_OBJDUMP ${TC_PATH}${CROSS_COMPILE}objdump
     CACHE FILEPATH "The toolchain objdump command " FORCE )
 
-set(COMMON_FLAGS "-fno-builtin -ffunction-sections -fdata-sections -fno-strict-aliasing -fmessage-length=0")
+set(COMMON_FLAGS "-fno-common -fno-builtin -ffunction-sections -fdata-sections -fno-strict-aliasing -fmessage-length=0")
 set(CMAKE_C_FLAGS "${COMMON_FLAGS} -std=gnu99")
 set(CMAKE_CXX_FLAGS "${COMMON_FLAGS} -std=gnu++0x")
 set(CMAKE_EXE_LINKER_FLAGS "-Wl,-gc-sections --specs=nosys.specs -nostdlib -static -nostartfiles")