Instruct macos users to use avr-gcc@7 (#3337)

If you run `brew install avr-gcc`, you get a version that has
compatibility issues with LUFA. I updated the getting started guide for
osx, the qmk_install setup script, and added a section to the FAQ for
folks like me who accidentally updated avr-gcc past 7.

docs/faq_build.md

@@ -104,3 +104,17 @@ brew install dfu-programmer
 brew install gcc-arm-none-eabi
 brew install avrdude
 ```
+
+### avr-gcc 8.1 and LUFA
+
+If you updated your avr-gcc to above 7 you may see errors involving LUFA. For example:
+
+`lib/lufa/LUFA/Drivers/USB/Class/Device/AudioClassDevice.h:380:5: error: 'const' attribute on function returning 'void'`
+
+For now, you need to rollback avr-gcc to 7 in brew.
+
+```
+brew uninstall --force avr-gcc
+brew install avr-gcc@7
+brew link avr-gcc@7
+```

docs/getting_started_build_tools.md

@@ -56,7 +56,7 @@ If you're using [homebrew,](http://brew.sh/) you can use the following commands:
     brew tap osx-cross/avr
     brew tap PX4/homebrew-px4
     brew update
-    brew install avr-gcc
+    brew install avr-gcc@7
     brew install dfu-programmer
     brew install gcc-arm-none-eabi
     brew install avrdude

util/macos_install.sh

@@ -22,4 +22,4 @@ fi
 brew tap osx-cross/avr
 brew tap PX4/homebrew-px4
 brew update
-brew install avr-gcc gcc-arm-none-eabi dfu-programmer avrdude
+brew install avr-gcc@7 gcc-arm-none-eabi dfu-programmer avrdude