CI: Reduce the min GCC version used to match what MakeCode (PXT) uses. (#99)

* CI: Reduce the min GCC version used to match what MakeCode (PXT) uses.

* Fix issue building in arm-none-eabi-gcc v6.

Including stdio.h before MicroBit.h caused a stdio macro for
putc(int, FILE*) to cause havoc on putc methods in multiple CODAL
classes like serial.

Author: microbit-carlos

.github/workflows/build.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-20.04, macos-13, windows-2019]
-        gcc: ['7-2017-q4', 'latest']
+        gcc: ['6-2016-q4', 'latest']  # Min version 6.2 as used in pext/yotta:latest docker image
         cmake: ['3.6.0', '']  # Empty string installs the latest CMake release
       fail-fast: false
     runs-on: ${{ matrix.os }}

source/samples/MicrophoneTest.cpp

@@ -1,10 +1,10 @@
-#include <stdio.h>
 #include "MicroBit.h"
 #include "SerialStreamer.h"
 #include "StreamNormalizer.h"
 #include "LevelDetector.h"
 #include "LevelDetectorSPL.h"
 #include "Tests.h"
+#include <stdio.h>
 
 static NRF52ADCChannel *mic = NULL;
 static SerialStreamer *streamer = NULL;