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.

1 files changed, 14 insertions, 0 deletions

diff --git a/docs/faq_build.md b/docs/faq_build.md
index 2d1b91b60..7d17df074 100644
--- a/docs/faq_build.md
+++ b/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
+```