display warnings (-Wall) for user code

It would be useful to display warnings (-Wall) for user code.
A lot of errors can be catched easily by turning it on.
New arduino version has also this option.

example program:

```C++
void setup()
{
    char y=555555;
    char z;
    char x=z;
}
void loop()
{
}
```

log:

```
src/src.cpp
.build/src/src.cpp: In function ‘void setup()’:
.build/src/src.cpp:4: warning: overflow in implicit constant conversion
.build/src/src.cpp:4: warning: unused variable ‘y’
.build/src/src.cpp:6: warning: unused variable ‘x’
.build/src/src.cpp:6: warning: ‘z’ is used uninitialized in this function
arduino/wiring_digital.c
arduino/wiring_shift.c
arduino/pins_arduino.c
arduino/wiring_analog.c
arduino/wiring.c
arduino/WInterrupts.c
arduino/wiring_pulse.c
arduino/Print.cpp
arduino/Tone.cpp
arduino/WMath.cpp
arduino/WString.cpp
arduino/HardwareSerial.cpp
arduino/main.cpp
Linking libcore.a
Linking firmware.elf
Converting to firmware.hex
```