Updated FAQ Build (markdown)

FAQ-Build.md

@@ -91,4 +91,14 @@ SUBSYSTEMS=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2ff0", MODE:="066
 ```
 # tmk keyboard products     https://github.com/tmk/tmk_keyboard
 SUBSYSTEMS=="usb", ATTRS{idVendor}=="feed", MODE:="0666"
-```
+```
+
+
+
+## Cortex: cstddef: No such file or directory
+GCC 4.8 of Ubuntu 14.04 had this problem and had to update to 4.9 with this PPA.
+https://launchpad.net/~terry.guo/+archive/ubuntu/gcc-arm-embedded
+
+https://github.com/tmk/tmk_keyboard/issues/212
+https://github.com/tmk/tmk_keyboard/wiki/mbed-cortex-porting#compile-error-cstddef
+https://developer.mbed.org/forum/mbed/topic/5205/